BANCROFT 

LIBRARY 

LIBRARY  CATALOGUE  SLIPS. 

United  States.      Department  of  the  interior.     (  U.  S.  geological  survey.) 
Department  of  the  interior  |  —  |  Monographs  |  of  the   |   United 

States  geological  survey   |   Volume  XX    |    [Seal  of  the  depart- 
ment] |  Washington  |  government  printing  office  |  1892 
Second  title:   United  States  geological  survey  |   J.  W.  Powell, 

director  |  —  |  Geology  |  of  the  |  Eureka  district  |  Nevada  |  with 

an  atlas  |  by  |  Arnold  Hague   |   [Vignette]  | 
Washington  |  government  printing  office  |  1892 
4°.    xvii,  419  pp.  8  pi. 


Hague  (Arnold). 

United  States  geological  survey  |  J.  W.  Powell,  director  | 
Geology  |  of  the  |  Eureka  district  |  Nevada  |  with  an  atlas  |   by  | 
Arnold  Hague  |  [Vignette]  | 

Washington  |  government  printing  office  |  1892 

4°.    xvii,  419pp.  8  pi. 

[UNITED    STATES.     Department  of  the  interior.     (U.    S.  geological  nuney). 

Monograph  XX.) 


£  United  States  geological  survey  |  J.  W.  Powell,  director  |  —  | 

I          Geology  |  of  the  |  Eureka  district  |  Nevada  |  with  an  atlus  |  by  | 

Arnold  Hague  |  [Vignette]  | 
?  Washington  |  government  printing  office  |  1892 

4°.    xvii,  419  pp.  8  pi. 
S  [UNITED  STATES.     Department  of   the  interior.     (17.  S.  geological  euney.) 

Monograph  XX.] 


[Monograph  XX.] 


The  publications  of  the  United  States  Geological  Survey  are  issued  in  accordance  with  the  statute 
approved  March  3,  1879,  which  declares  l  hat — 

"The  publications  of  the  Geological  Survey  shall  consist  of  the  annual  report  of  operations,  geo- 
logical and  economic  maps  illustrating  the  resources  and  classification  of  the  lands,  a.nd  reports  upon 
general  and  economic  geology  and  paleontology.  The  annual  report  of  operations  of  the  Geological 
Survey  shall  accompany  the  animal  report  of  the  Secretary  of  the  Interior.  All  special  memoirs  and 
reports  of  said  Survey  shall  be  issued  in  uniform  quarto  series  if  deemed  neceasary  by  the  Director,  hut 
otherwise  iii  ordinary  octavos.  Three  thousand  copies  of  each  shall  be  published  lor  scientific  exchanges 
and  for  sale  at  the  price  of  publication;  and  all  literary  and  cartographic  materials  received  in  exchange 
shall  be  the  property  of  the  United  States  and  form  a  part  of  the  library  of  the  organization :  And  the 
money  resulting  from  the  sale  of  such  publications  shall  be  covered  into  the  Treasury  of  the  United 
States." 

The  following  joint  resolution,  referring  to  all  government  publications,  was  passed  by  Congress 
July  7,  1882: 

"That,  whenever  any  document  or  report,  shall  be  ordered  printed  by  Congress,  there  shall  be 
printed,  in  addition  to  the  number  in  each  case  stated,  the  'usual  number'  (1,900)  of  copies  for  binding 
and  distribution  among  those  entitled  to  receive  them." 

Except  in  those  cases.in  which  an  extra  number  of  any  publication  has  been  supplied  to  the  Sur- 
vey by  special  resolution  of  Congress  or  has  been  ordered  by  the  Secretary  of  the  Interior,  this  office 
has  no  copies  for  gratuitous  distribution. 

ANNUAL  REPORTS. 

I.  First  Annual  Report  of  the  United  States  Geological  Survey,  by  Clarence  King.    1880.    8°.    79 
pp.    1  map. — A  preliminary  report  describing  plan  of  organization  and  publications. 

II.  Second  Annual  Report  of  the  United  States  Geological  Survey,  1880-'81,  bv  J.  W.  Powell. 

1882.  8C.     Iv,  588  pp.    62  pi.     1  map. 

III.  Third  Annual  Report  of  the  United  States  Geological  Survey,  1881-'82,  by  J.  W.  Powell. 

1883.  8°.     xviii,  564  pp.     67  pi.  and  maps. 

IV.  Fourth  Annual  Report  of  the  United  States  Geological  Survey,  1882-'83,  by  J.  W.  Powell. 

1884.  8°.    xxxii,  473  pp.    85  pi.  and  maps. 

V.  Fifth  Annual  Report  of  the  United  States  Geological  Survey,  1883-'84,  by  J.  W.  Powell. 
1385.     8°.    xxxvi,  469  pp.     58  pi.  and  maps. 

VI.  Sixth  Annual  Report  of  the  United  States  Geological  Survey,  1884-'85)  by  J.  W.  Powell. 

1885.  8°.    xxix,  570  pp.    66  pi.  and  maps. 

VII.  Seventh  Annual  Report  of  the  United  States  Geological  Survey,  1885-'86,  by  J.  W.  Powell. 

1888.  8°.     xx,  656pp.    71  pi.  and  maps. 

VIII.  Eighth  Annual  Report  of  the  United  States  Geological  Survey,  1886-'87,  by  J.  W.  Powell. 

1889.  8°.     2v.     xix,  474,  xii  pp.    5:!  pi.  and  maps;  1  p.  1.     475-1063  pp.     54-76  pi.  and  maps, 

IX.  Ninth  Annual  Report  of  the  United  States  Geological  Survey,  1887-'88,  by  J.  W.  Powell. 

1889.  8°.     xiii,  717  pp.     88  pi.  and  maps. 

X.  Tenth  Annual  Report  of  the  United  States  Geological  Survey,  1888-'89,  by  J.  W.  Powell. 

1890.  8°.    2  v.     xv, 774  pp.    98  pi.  and  maps;  viii,  123  pp. 

XI.  EleventhJAnnual  Report  of  the  United  States  Geological  Survey,  1889-'90,  by  J.  W.  Powell. 

1891.  8°.     2  v.  xv,  757  pp.     66  pi.  and  maps  ;  ix,  3">1  pp.     30  pi.  and  maps. 

XII.  Twelfth  Annual  Report  of  the  United  States  Geological  Survey,  1890-'!U,  by  J.  W.  Powell. 
1891.     8°    2  v.    xiii,  675  pp.     53  pi.  and  maps  ;  xviii,  576  pp.     146  pi.  aud  maps. 

MONOGRAPHS. 

I.  Lake  Bonneville,  by  Grove  Karl  Gilbert.    1890.    4°.    xx,  438  pp.    51  pi.    1  map.    Price  $1.50. 

II.  Tertiary  History  of  the  Grand  Canon  District,  with  atlas,  by  Clarence  E.  Button,  Capt.,  U.  S.  A. 
1882.    4°.     xiv,  264  pp.     42  pi.  and  atlas  of  24  sheets  folio.     Price  $10.00. 

III.  Geology  of  the  Comstock  Lode  and  the  Washoe  District,  with  atlas,  by  George  F.  Becker. 
1882.    4°.     xv,  422pp.     7  pi.  and  atlas  of  21  sheets  folio.     Price  $11.00. 

IV.  Comstock  Mining  and  Miners,  by  Eliot  Lord.     1883.    4°.     xiv,  451  pp.     3  pi.     Price  $1.50. 

I 


II  ADVERTISEMENT. 

V.  The  Copper-Bearing  Rocks  of  Lake  Superior,  by  Rolanil  Duer  Irving.    1883.    4°.    xvi,464pp. 
I'll.     29  pi.  and  maps.     Pricefl.85. 

VI.  Contributions  to  the  Knowledge  of  the  Older  Mesozoic  Flora  of  Virginia,  by  William  Morris 
Fontaine.     188.3.     4°.     xi,  144  pp.     54 1.     54  pi.     Price  $1.05. 

VII.  Silver-Lead  Deposits  of  Eureka,  Nevada,  by  Joseph  Story  Curtis.     1884.     4°.     xiii,  200  pp. 
16  pi.     Price  $1.20. 

VIII.  Paleontology  of  the  Eureka  District,  by  Charles  Doolittle  Walcott.    1884.    4°.    xiii,29S  pp. 
241.    24  pi.     Price  $1.10. 

IX.  Brachiopoda  and  Lamellibrancbiata  of  the  Raritan  Clavs  and  Greensand  Marls  of  New  Jersey, 
by  Robert  P.  Whittield.     1885.     4°.     xx,  338  pp.     35  pi.     1  map.     Price  81. 15. 

X.  Diuocerata.     A  Monograph  of  an  Extinct  Order  of  Gigantic  Mammals,  by  Othniel  Charles 
Marsh.     1886.     4°.     xviii,  243pp.     561.     56  pi.    Price  $2.70. 

XI.  Geological  History  of  Lake  Lahontan,  a  Quaternary  Lake  of  Northwestern  Nevada,  by  Israel 
Cook  Russell.     1&85.     4°.     xiv,  288  pp.     46  pi.  and  maps.     Price  $1.75. 

XII.  Geology  and  Mining  Industry  of  Leadville,  Colorado,  with  atlas,  by  Samuel  Franklin  Em- 
inons.     1886.     4°.     xxix,  770  pp.     45  pi.  and  atlas  of  35  sheets  folio.    Price  $8.40. 

XIII.  Geology  of  the  Quicksilver  Deposits  of  the  Pacific  Slope,  with  atlas,  by  George  F.  Becker. 
1888.     4°.     xix,  486  pp.     7  pi.  and  atlas  of  14  sheets  folio.     Price  $2.00. 

XIV.  Fossil  Fishes  and  Fossil  Plants  of  the  Triassie  Rocks  of  New  Jersey  and  the  Connecticut 
Valley,  by  John  S.  Newberry.     1888.     4°.     xiv,  152  pp.    2(5  pi.     Price  $1.0H. 

XV.  The  Potomac  or  Younger  Mesozoic  Flora,  by  William  Morris  Fontaine.     1889.     4°.     xiv, 
377  pp.     180  pi.     Text  and  plates  bound  separately.     Price  $2.50. 

XVI.  The  Paleozoic  Fishes  of  North  America,   by  John  Strong  Newberry.     1889.     4°.     340  pp. 
53  pi.     Price  $1.00. 

XVII.  The  Flora  of  the  Dakota  Group,  a  posthumous  work,  by  Leo  Lesquereux.     Edited  by  F.  H. 
Knowlton.     1891.    4°.    400pp.    66  pi.     Price$t.lO. 

XVIII.  Gasteropoda  and  Cephalopoda  of  the  Raritan  Clays  and  Greensand  Marls  of  New  Jersey, 
by  Robert  P.  Whittield.     1891.     4°.     402  pp.     50  pi.     Price  $1.00. 

XX.  Geology  of  the  Eureka  District,  Nevada,  with  an  atlas,  by  Arnold  Hague.     1892.     4°.     xvii, 
419  pp.     8  pi.     Price  $5.25. 

In  press : 

XIX.  The  Penokee  Iron-Bearing  Series  of  Northern  Wisconsin  and  Michigan,  by  Roland  D. 
Irving  and  C.  R.  Van  Hise. 

XXI.  The  Tertiary  Rhynchophorous  Coleoptera  of  North  America,  by  S.  H.  Scudder. 

XXII.  Geology  of  the  Green  Mountains  in  Massachusetts,  by  Messrs.  Punipelly,  Wolff,  and  Dale. 

In  preparation : 

— Mollusca  and  Crustacea  of  the  Miocene  Formations  of  New  Jersey,  by  R.  P.  Whitfleld. 

— Sauropoda,  by  O.  C.  Marsh. 

— Stegosauria,  by  O.  C.  Marsh. 

— Brontotheridae,  by  O.  C.  Marsh. 

— Report  on  the  Denver  Coal  Basin,  by  S.  F.  Eminons. 

— Report  on  Silver  Cliff  and  Ten-Mile  Mining  Districts,  Colorado,  by  S.  F.  Eiumons. 

— The  Glacial  Lake  Agassiz,  by  Warren  Upham. 

BULLETINS. 

1.  On  Hypersthene-Andcsite  and  on  Triclinic  Pyroxene  in  Augitic  Rocks,  by  Whitman  Cross,  with 
a  Geological  Sketch  of  Buffalo  Peaks,  Colorado,  by  S.  F.  Einmons.    1883.    8°.    42  pp.    2  pi.     Price  10 
cents. 

2.  Gold  and  Silver  Conversion  Tables,  giving  the  coining  values  of  troy  ounces  of  fine  metal,  etc., 
computed  by  Albert  Williams,  jr.     1883.     8°.    8  pp.     Price  5  cents. 

3.  On  the  Fossil  Faunas  of  the  Upper  Devonian,  along  the  meridian  of  76°  30',  from  Tompkins 
County,  N.  Y.,  to  Bradford  County,  Pa.,  by  Henry  S.  Williams.     1884.     8°.     36pp.    Price  5  ceuts. 

4.  On  Mesozoic  Fossils,  by  Charles  A.  White.     1884.     8°.     36  pp.     9  pi.     Price  5  cents. 

5.  A  Dictionary  of  Altitudes  in  the  United  States,  compiled  by  Henry  Gannett.    1884.    8°.    325pp. 
Price  20  cents. 

6.  Elevations  in  the  Dominion  of  Canada,  by  J.  W.  Spencer.     1884.     8°.     43  pp.     Price  5  cents. 

7.  Mapoteca  Geologica  Americana.    A  Catalogue  of  Geological  Maps  of  America  (North  and  South), 
1752-1881,  in  geographic  and  chronologic  order,  by  Jules  Marcou  and  John  Belknap  Marcou.     1884. 
8°.     184  pp.     Price  10  cents. 

8.  On  Secondary  Enlargements  of  Mineral  Fragments  in  Certain  Rocks,  by  R.  D.  Irving  and  C.  R. 
Van  Hise.     1884.    8°.    56  pp.     6  pi.     Price  10  cents. 

9.  A  Report  of  work  done  in  the  Washington  Laboratory  during  the  fiscal  year  1883-'84.     F.  W. 
Clarke,  chief  chemist;  T.  M.  Chatard,  assistant  chemist.     1884.     8°.     40pp.     Price  5  cents. 

10.  On  the  Cambrian  Faunas  of  North  America.     Preliminary  studies,  by  Charles  Doolittle  Wal- 
cott.    1884.     8°.    74  pp.     10  pi.     Price  5  cents. 

11.  On  the  Quaternary  and  Recent  Mollusca  of  the  Great  Basin  ;  with  Descriptions  of  New  Forms, 
by  R.  Ellsworth  Call.     Introduced  by  a  sketch  of  the  Quaternary  Lakes  of  the  Great  Basin,  by  G.  K. 
G'ilbert.     1884.     8°.     66  pp.     6  pi.     Price  5  cents. 


ADVERTISEMENT.  Ill 

12.  A  Crystallographic  Study  of  the  Thinolite  of  Lake  Lahontan,  by  Edward  S.  Dana.     1884.     8°. 
34  pp.     3  pi.     Price  5  cents. 

13.  Boundaries  of  the  United  States  and  of  the  several  States  and  Territories,  with  a  Historical 
Sketch  of  the  Territorial  Changes,  by  Henry  Gannett.     1H85.     8°.     135pp.     Price  10  cents. 

14.  The  Electrical  and  Magnetic  Properties  of  the  Iron-Carburets,  by  Carl  Barns  and  Vincent 
Stronhal.     1885.     8°.     238  pp.     Price  15  cents. 

15.  On  the  Mesozoic  and  Cenozoic  Paleontology  of  California,  by  Charles  A.  White.     1885.     8°. 
33  pp.     Price  5  cents. 

16.  On  the  Higher  Devonian  Faunas  of  Ontario  County,  New  York,  by  John  M.Clarke.     I88a.     8°. 

86pp.     3  pi.     Price  5  cents. 

17.  On  the  Development  of  Crystallization  in  the  Igneous  Rocks  of  Washoe,  Nevada,  with  Notes 
on  the  Geology  of  the  District,  by  Arnold  Hague  and  Joseph  P.  Iddings.     1885.     8°.     44  pp.     Price  5 
cents. 

18.  On  Marine  Eocene,  Fresh-water  Miocene,  and  other  Fossil  Mollusca  of  Western  North  America, 
by  Charles  A.  White.     1885.     8°.     26  pp.     3  pi.     Price  5  cents. 

19.  Notes  on  the  Stratigraphy  of  California,  by  George  F.Becker.    1885.    8°.    28pp.    Price5cents. 

20.  Contributions  to  the  Mineralogy  of  the  Rocky  Mountains,  by  Whitman  Cross  and  W.  F.  Hille- 
brand.     1885.     8°.     114  pp.     1  pi.     Price  10  cents. 

21.  The  Lignites  of  the  Great  Sioux  Reservation.     A  Report  on  the  Region  between  the  Grand  and 
Moreau  Rivers,  Dakota,  by  Bailey  Willis.     1885.    8°.     16pp.     5  pi.     Price  5  cents. 

22.  On  New  Cretaceous  Fossils  from  California,  by  Charles  A.  White.     1885.     8°.     25  pp.     5  pi. 
Price  5  cents. 

23.  Observations  on  the  Junction  between  the  Eastern  Sandstone  and  the  Keweenaw  Series  on 
Keweenaw  Point,  Lake  Superior,  by  R.  D.  Irving  and  T.  C.  Chamberlin.     1885.     8°.     124  pp.     17  pi. 
Price  15  cents. 

24.  List  of  Marino  Mollusca,  comprising  the  Quaternary  fossils  and  recent  forms  from  American 
Localities  between  Cape  Hatteras  and  Cape  Roqne,  including  the  Bermudas,  by  William  Healey  Dall. 
1885.     8C.     336  pp.     Price  25  cents. 

25.  The  Present  Technical  Condition  of  the  Steel  Industry  of  the  United  States,  by  Phineas  Barnes. 
1885.     8°.     85  pp.     Price  10  cents. 

26.  Copper  Smelting,  by  Henry  M.  Howe.     1885.     8°.     107  pp.     Price  10  cents. 

27.  Report  of  work  done  in  the  Division  of  Chemistry  and  Physics,  mainly  during  the  fiscal  year 
1884-'85.     1886.     8°.     80  pp.     Price  10  cents. 

28.  TheGabbros  and  Associated  Hornblende  Rocks  occurring  in  the  Neighborhood  of  Baltimore, 
Md.,  oy  George  Huntingtou  Williams.     1886.     8°.     78pp.    4  pi.     Price  10  cents. 

29.  On  the  Fresh- water  Invertebrates  of  the  North  American  Jurassic,  by  Charles  A.  White.     1886, 
8°.     41  pp.     4  pi.     Price  5  cents. 

30.  Second  Contribution  to  the  Studies  on  the  Cambrian  Faunas  of  North  America,  by  Charles 
Doolittle  Walcott.     1H86.     8°.     369  pp.     33  pi.     Price  25  cents. 

31.  Systematic  Review  of  our  Present  Knowledge  of  Fossil  Insects,  including  Myriapods  and 
Arachnids,  by  Samuel  Hubbard  Scudder.     1886.     8°.     128  pp.     Price  15  cents. 

32.  Lists  and  Analyses  of  the  Mineral  Springs  of  the  United  States;  a  Preliminary  Study,  by 
Albert  C.  Peale.     1886.     8°.    235  pp.     Price  20  cents. 

33.  Noteson  the  Geology  of  Northern  California,  by  J.  S.DHler.     1886.     8°.    23pp.    Priceocents. 

34.  On  the  relation  of  the  Laramie  Molluscan  Fauna  to  that  of  the  succeeding  Fresh- water  Eocene 
and  other  groups,  by  Charles  A.  White.     1886.    8°.     54  pp.     5  pi.     Price  10  cents. 

35.  Physical  Properties  of  the  Iron-Carburets,  by  Carl  Barns  and  Vincent  Strouhal.     1886.    8°. 
62  pp.    Price  10  cents. 

36.  Subsidenceof  Fine  Solid  Particles  in  Liquids,  byC'arlBarus.    1886.    8°.    58pp.    Price  10  cents. 

37.  Types  of  the  Lanimie  Flora,  by  Lester  F.  Ward.     1887.     8°.     354pp.     57  pi.     Price  25  cents. 

38.  Peridotite  of  Elliott  County,  Kentucky,  by  J  S.  Diller.    1887.    8°.    31pp.    1  pi.    Price  5  cents. 

39.  The  Upper  Beaches  and  Deltas  of  the  Glacial  Lake  Agassiz,  by  Warren  Upham.     1887.     8°. 
84  pp.     1  pi.     Price  10  cents. 

40.  Changes  in  River  Courses  ia  Washington  Territory  due  to  Glaciation,  by  Bailey  Willis.    1887. 
8°.     10  pp.     4  pi.     Price  5  cents. 

41.  On  the  Fossil  Faunas  of  the  Upper  Devonian— the  Geuesee  Section,  New  York,  by  Henry  S. 
Williams.     1887.     8°.     121  pp.     4  pi.     Price  15  cents. 

42.  Report  of  work  done  in  the  Division  of  Chemistry  and  Physics,  mainly  during  the  fiscal  year 
1885-'86.     F.  W.  Clarke,  chief  chemist.     1887.     8°.     152  pp.     1  pi.     Price  15  cents. 

43.  Tertiary  and  Cretaceous  Strataof  the  Tuscaloosa,  Tombigbee,  and  Alabama  Rivers,  by  Eugene 
A.  Smith  and  Lawrence  C.Johnson.     1887.     8U.     189pp.    21  pi.     Price  15  cents. 

44.  Bibliography  of  North  American  Geology  for  1886,  by  Nelson  H.  Dartou.     1887.     8°.     35  pp. 
Price  5  cents. 

45.  The  Present  Condition  of  Knowledge  of  the  Geology  of  Texas,  by  Robert  T.  Hill.     1887.     8°. 
94  pp.     Price  10  cents. 

46.  Nature  and  Origin  of  Deposits  of  Phosphate  of  Lime,  by  R.  A.  F.  Penrose,  jr.,  with  an  Intro- 
duction by  N.  S.  Shaler.     1888.     8°.     143  pp.     Price  15  cents. 

47.  Analyses  of  Waters  of  the  Yellowstone  National  Park,  with  an  Account  of  tin-  Methods  of 
Analysis  employed,  by  Frank  Austin  Gooch  and  James  Edward  Whitfield.     1888.     8°.     84  pp.     Price 

10  cents. 

48.  On  the  Form  and  Position  of  the  Sea  Level,  by  Robert  Simpson  Woodward.     1888.    b°.    88 

pp.     Price  10  cents. 


IV  ADVERTISEMENT. 

49.  Latitudes  and  Longitudes  of  Certain  Points  in  Missouri,  Kansas,  and  New  Mexico,  by  Robert 
Simpson  Woodward.     1889.     8°.     133  pp.     Price  15  cents. 

50.  Formulas  and  Tables  tu  facilitate  the  Construction  and  Use  of  Maps,  by  Robert  Simpson 
Woodward.     1889.     8C.     124  pp.     Price  15  cents. 

51.  On  Invertebrate  Fossils  from  the  Pacific  Coast,  by  Charles  Abiatliar  Wbite.     1889.     8C.     102 
pp.     14  pi.     Pric-e  15  cents. 

52.  Subae'rial  Decay  of  Rocks  and  Origin  of  the  Red  Color  of  Certain  Formations,  by  Israel  Cook 
Russell.     18>9.     ,-  .     65pp.    5  pi.     Price  10  cents. 

63.  The  Geology  of  Nantncfcet,  by  Nathaniel  SonthgateShaler.    1889.    8°.    55pp.    10  pi.    Price 
10  cents. 

54.  On    the    Thermo-Electric  Measurement  of  High  Temperatures,  by   Carl  Barns.     1889.     8°. 
313pp.  iucl.  1  pi.    11  pi.     Price  25 "cents. 

55.  Report  of  work  done  in  the  Division  of  Chemistry  and  Physics,  mainly  during  the  fiscal  jcar 
1886-'87.     Frank  Wigglesworth  Clarke,  chief  chemist.     1889.     8°.  '96pp.     Price  10  cents. 

56.  Fossil  Wood  and  Lignite  of  the  Potomac  Formation,  by  Frank  Hall  Ktiowlton.     1889.     s  . 
72  pp.    7  pi.     Price  10  cents. 

57.  A  Geological  Reconnaissance  in  Southwestern  Kansas,  by  Robert  Hay.     1890.     8°.     49  pp. 
2  pi.     Price  5  cents. 

58.  The  Glacial  Boundary  in  Western  Pennsylvania,  Ohio,  Kentucky,  Indiana,  and  Illinois,  by 
George  Frederick  Wright,  with  an  introduction  by  Thomas  Chrowder  Chamberlin.     1890.     8°.     112 
pp.  incl.  1  pi.     8  pi.     Price  15  cents. 

59    The  Gabbros  and  Associated  Rocks  in  Delaware,  by  Frederick  D.Chester.     1890.     8°.     45pp. 
1  pi.     Price  10  cents. 

60.  Report  of  work  done  in  the  Division  of  Chemistry  and  Physics,  mainly  during  the  fiscal  year 
1887-'88.     F.  W.  Clarke,  chief  chemist.     1890.     8°.     174  pp'.     Price  15  cents. 

61.  Contributions  to  the  Mineralogy  of  the  Pacific  Coast,  by  William  Harlow  Melville  and  Wal- 
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62.  The  Greenstone  Schist  Areas  of  the  Meuominee  and  Marqnette  Regions  of  Michigan,  a  contri- 
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with  an  introduction  by  Roland  Duer  Irving.     1>90.     8°.     241  pp.     16  pi.     Price  30  cents. 

63.  A  Bibliography  of  Paleozoic  Crustacea  from  1698  to  1889,  including  a  list  of  North  Amer- 
ican species  and  a  systematic  arrangement  of  genera,  by  Anthony  W.  Vogdes.     18!tO.     8°.     177  pp. 
Price  15  cent*. 

64.  A  Report  of  work  done  in  the  Division  of  Chemistry  and  Physics,  mainly  during  the  fiscal 
year  1888-'89.     F.  W.  Clarke,  chief  chemist.    1890.     8°.     60pp.     Price  10  cents. 

65.  Stratigraphy  of  the  Bituminous  Coal  Field  of  Pennsylvania,  Ohio,  and  West  Virginia,  by 
Israel  C.  White.     1891.     8°.     21<i  pp.     11  pi.     Price  20  cents. 

66.  On  a  Group  of  Volcanic  Rocks  from  the  Tewan  Mountains,  New  Mexico,  and  on  the  occurrence 
of  Primary  Quartz  in  certain  Basalts,  by  Joseph  Paxson  Iddings.     1890.     8°.     34  pp.     Price  5  cents. 

67.  The  relations  of  the  Traps  of  the  Newark  System  iu  the  New  Jersey  Region,  by  Nelson  Horatio 
Darton.     1*90.     8°.     82  pp.     Price  10  cents. 

68.  Earthquakes  in  California  in  li-89,  by  James  Edward  Keeler.    1890.    8°.    25pp.    Price5  cents. 

69.  A  Classed  and  Annotated  Bibliography  of  Fossil  Insects,  by  Samuel  Hubbard  Scudder.     1890. 
8°.     101  pp.     Price  15  cents. 

70.  Report  on  Astronomical  Work  of  1889  and  1890,  by  Robert  Simpson  Woodward.     1890.     8°. 
79pp.     Price  10  cents. 

71.  Index  to  the  Known  Fossil  Insects  of  the  World,  including  Myriapods  and  Arachnids,  by 
Samuel  Hubbard  Scudder.     1891.     8°.    744pp.     Price  50  cents. 

72.  Altitudes  between  Lake  Superior  and  the  Rocky  Mountains,  by  Warren  Upham.     1891.    8°. 
229  pp.     Price  20  cents. 

73.  The  Viscosity  of  Solids,  by  Carl  Barus.     1891.     8°.     xii,  139  pp.     6  pi.     Price  15  cents. 

74.  The  Minerals  of  North  Carolina,  by  Frederick  Augustus  Genth.     1891.     8°.     119pp.     Price 
15  cents. 

75.  Record  of  North  American  Geology  for  1887  to  1889,  inclusive,  by  Nelson  Horatio  Darton. 
1891.     8°.     173  pp.     Price  15  cents. 

76.  A  Dictionary  of  Altitudes  in  the  United  States  (second  edition),  compiled  by  Henry  Gannett, 
chief  topographer.     1891.     8°.    393pp.     Price  25  cents. 

77.  The  Texan  Permian  and  its  Mesozoic  types  of  Fossils,  by  Charles  A.  White.     1891.     8°.     51 
pp.     4  pi.     Price  10  cents. 

78.  A  report  of  work  done  in  the  Division  of  Chemistry  and  Physics,  mainly  during  the  fiscal  year 
1889-'90.     F.  W.  Clarke,  chiet  chemist.     IS'.lt.     8°.     131  pp.     Price  15  cents. 

79.  A  Late  Volcanic.  Eruption  in  Northern  California  and  its  peculiar  lava,  by  J.  S.  Diller. 

80.  Correlation   papers— Devonian  and  Carboniferous,  by  Henry  Shaler  Williams.     1891.    8°. 
279  pp.     Price  20  cents. 

81.  Correlation    papers — Cambrian,    by   Charles   Doolittle  Walcott.     1S91.     8°.     447  pp.     3  pi. 
Price  25  cents. 

82.  Correlation  papers— Cretaceous,   by  Charles  A.   White.     1891.    8°.     273  pp.    3  pi.     Price 
20  cents. 

83.  Correlation  papers — Eocene,  by  William  Bullock  Clark.     1891.    8°.    173pp.    2  pi.    Price  15 
cents. 

91.  Record  of  North  American  Geology  for  1890,  by  Nelson  Horatio  Darton.     1891.     8°.     88  pp. 
Price  10  cents. 


ADVERTISEMENT.  V 

In  press: 

84.  Correlation  papers — Neocene,  by  W.  H.  Dall  and  G.  D.  Harris. 

85.  Correlation  papers — Tlie  Newark  System,  by  I.  (J.  Knssell. 

86.  Correlation  papers — Algonkian  and  Archean,  by  C.  R.  Van  Hise. 

87.  Bibliography  and  Index  of  the  publications  of  the  U.  S.  Geological  Survey,  1879-1892,  by 
P.  C.  Wannan. 

!»().  A  report  of  work  done  in  the  Division  of  Chemistry  and  Physics,  mainly  during  the  fiscal 
year  1890-'91.  F.  W.  Clarke,  chief  chemist. 

92.  The  Compressibility  of  Liquids,  by  Carl  Barns. 

93.  Some  Insects  of  special  interest  from  Florissant,  Colorado,  by  K.  II.  Scudder. 
91.  The  Mechanism  of  Solid  Viscosity,  by  Carl  Barus. 

95.  Earthquakes  in  California  during  1890-'91,  by  E.  S.  Holden. 

96.  The  Volume  Thermodynamics  of  Liquids,  by  Carl  Barns. 

97.  The  Mesozoic  Echinodermata  of  the  United  States,  by  W.  B.  Clark. 

98.  Flora  of  the  Outlying  Coal  Basins  of  Southwestern  Missouri,  by  David  White. 

99.  Record  of  North  American  Geology  for  1891,  by  Nelson  Horatio  Dartou. 

In  preparation  : 

88.  Correlation  papers — Pleistocene,  by  T.  C.  Chamberlin. 

100.  The  Eruptive  and  Sedimentary  Rocks  on  Pijreon  Point,  Minnesota,  and  their  contact  phe- 
nomena, by  W.  S.  Bayley. 

101.  Insect  fauna  of  the  Rhode  Island  Coal-licld,  by  Samuel  Hubbard  Scudder. 

102.  A  Catalogue  and  Bibliography  of  North  American  Mesozoic  Invertebrata,  by  C.  B.  Boyle. 

103.  The  Trap  Dikes  of  Lake  Champlain  Valley  and  the  Eastern  Adirondacks,  by  J.  F.  Kemp. 

—  High  Temperature  Work  in  Igneous  Fusion  and  Ebullition,  chiefly  in  relation  to  Pressure, 
by  Carl  Barus. 

—  Glaciatiou  of  the  Yellowstone  Valley,  by  W.  H.  Weed. 

—  The  Laramie  and  the  overlying  Livingstone  Formation  in  Montana,  by  W.  H.  Weed,  with 
Report  on  Flora,  by  F.  H.  Knowlton. 

—  The  Moraines  of  the  Missouri  Coteau  and  their  attendant  deposits,  by  James  Edward  Todd. 

—  A  Bibliography  of  Paleobotany,  by  David  White. 

STATISTICAL  PAPERS. 

Mineral  Resources  of  the  United  States  [1882],  by  Albert  Williams,  jr.  1883.  8°.  xvii,  813  pp. 
Price  50  cents. 

MiniM-al  Resources  of  the  United  States,  1883  and  1884,  by  Albert  Williams,  jr.  1885.  8°.  xiv, 
1016  pp.  Price  60  cents. 

Mineral  Resources  of  the  United  States,  1885.  Division  of  Mining  Statistics  and  Technology. 
1886.  8°.  vii,  576  pp.  Price  40  cents. 

Mineral  Resources  of  the  United  States,  1886,  by  David  T.  Day.  1887.  8°.  viii,  813  pp.  Price 
50  cents. 

Mineral  Resources  of  the  United  States,  1887,  by  David  T.  Day.  1888.  8°.  vii,  832  pp.  Price 
50  cents. 

Mineral  Resources  of  the  United  States,  1888,  by  David  T.  Day.  1890.  8°.  vii,  652  pp.  Price 
50  cents. 

Mineral  Resources  of  the  United  States,  1889  and  1890,  by  David  T.  Day.  1892.  8°.  viii.  671  pp. 
Price  50  cents. 

The  money  received  from  the  sale  of  these  publications  is  deposited  in  the  Treasury,  and  the 
Secretary  of  that  Department  declines  to  receive  bank  checks,  drafts,  or  postage-stamps;  all  remit- 
tances, therefore,  must  be  by  POSTAL  NOTE  or  MONEY  ORDER,  made  payable  to  the  Librarian  of  the 
U.  S.  Geological  Survey,  or  in  CURRENCY  for  the  exact  amount.  Correspondence  relating  to  the  pub- 
lications of  the  Survey  should  be  addressed 

To  THE  DIRECTOR  OF  THE 

UNITED  STATES  GEOLOGICAL  SURVEY, 

WASHINGTON,  D.  C. 
WASHINGTON,  D.  C.,  September,  1892. 


DEPARTMENT    OF    THE    INTERIOR 


MONOGRAPHS 


OF  THE 


UNITED  STATES  GEOLOGICAL  SURVEY 


VOLUME   XX 


WASHINGTON 

GOVKKNMENT     PRINTING     OFFICK 
1892 


UNITED   STATES   GEOLOGICAL   SURVEY 

J.    W.  POWELL,  DIRECTOR 


GEOLOGY 


OF  THE 


EUREKA    DISTRICT,  NEVADA 


WITH    AN    ATLAS 


BY 


WASHINGTON 

GOVERNMENT  PRINTING  OFFIOK 
1892 

For  Atlas 

see 

Map  Collection 
F847 
.89 
1883 
.H13 


F 


|54I5 

Bancroft  Library 


CONTENTS. 


LETTER  OF  TRANSMITTAL ix 

PREFACE xi 

OUTLINE  OF  THIS  VOLUME xv 

CHAPTER       I. — General  description 1 

CHAPTER     II. — Geological  sketch  of  the  Eureka  District 8 

CHAPTER   III. — Cambrian  and  Silurian  rocks 34 

CHAPTER    IV. — Devonian  and  Carboniferous  rocks 63 

CHAPTER      V. — Descriptive  geology 99 

CHAPTER    VI. — General  discussion  of  the  Paleozoic  rocks 175 

CHAPTER  VII. — Pro-Tertiary  igneous  rocks 218 

CHAPTER  VIII. — Tertiary  and  post-Tertiary  volcanic  rocks 230 

CHAPTER     IX. — Ore  deposits 292 

APPENDIX     A. — Systematic  lists  of  fossils  of  each  geological  horteon.     By  C.  D.  Walcott 317 

APPENDIX     B. — Microscopical  petrography  of  the  eruptive  rocks.     By  .1.  P.  Iddings 335 

INDEX 407 

v 


ILLUSTRATIONS. 


Page. 

PLATE      I.  Geological  Map  of  Ruby  Hill 115 

II.  Geological  Cross-sections 174 

III.  Minerals  in  section 396 

IV.  Quartz  grains  in  section 398 

V.  Feldspars  in  section 400 

VI.  Micropegmatite  and  andesite 402 

VII.  Andesite  and  basalt 404 

VIII .  Rhyolite 406 

FIG.  1.  Silurian  quartzite  west  of  Castle  Mountain 56 

2.  Section  across  Lone  Mountain 62 

3.  Nevada  limestone,  Modoc  Section 66 

4.  Section  across  Pifion  Range 201 

5.  Granite  porphyry  dikes 223 

6.  Vertical  Cross-section — Phoenix  mine 306 

7.  Carlsbad  twinn  of  labradorite 351 

8.  Diagram  exhibiting  extinction  angles  of  a  Carlsbad  twin  of  Inbradorite 351 

9.  Carlsbad  twin  of  plagioclase 353 

VII 


ATLAS  SHEETS. 


Title Sheet  I. 

List  of  Atlas  Sheets  and  Legends Sheet  II. 

Topographical  and  Index  Map  of  the  Kureka  I  >istrict Sheet  III. 

Geological  Map  of  the  Eureka  District   - Sheet  IV. 

Geological  Map  of  Northwest  Sheet Sheet  V. 

Geological  Map  of  Northeast  Sheet Sheet  VI. 

Geological  Map  of  Northwest-Cent™  1  Sh.'c-t Sheet  VII. 

Geological  Map  of  Northeast-Central  Sheet Sheet  VIII. 

Geological  Map  of  Southwest-Central  Sheet Sheet  IX. 

Geological  Map  of  Southeast-Central  Sheet Sheet  X. 

Geological  Map  of  Southwest  Sheet .' Sheet  XI. 

Geological  Map  of  Southeast  Sheet Sheet  XII. 

Geological  Cross-sections Sheet  XIII. 

VIII 


LETTER  OF  TRANSMITTAL. 


DEPARTMENT  OF  THE  INTERIOR, 
UNITED   STATES  GEOLOGICAL  SURVEY, 

Washington,  I).  C.,  June  30,  1891. 

SIR  :  I  have  the  honor  to  transmit  herewith  a  report  on  the  Geology  of 
the  Eureka  District,  Nevada. 

To  yourself  and  to  the  Hon.  Clarence  King,  under  whose  direction  the 
field  work  was  commenced,  I  am  greatly  indebted  for  the  personal  interest 
taken  in  the  investigation,  and  also  for  the  generous  facilities  afforded  me 
both  in  the  field  and  office. 

Very  respectfully,  your  obedient  servant, 

ARNOLD  HAGUE, 

Geologist  in  Charge. 
Hon.  J.  W.  POWELL, 

Director,  U.  S.  Geological  Survey. 


PREFACE. 


The  survey  of  the  Eureka  District  was  authorized  by  the  Hon.  Clarence 
King,  the  first  Director  of  the  United  States  Geological  Survey,  and  the 
field  work,  for  the  most  part,  was  done  during  his  administration.  The  field 
season  was  confined  to  the  summer  and  autumn  of  1880,  and  was  limited  to 
five  months,  the  work  being  brought  abruptly  to  a  close  early  in  December 
owing  to  the  inclemency  of  the  weather.  Visits  of  a  few  days'  duration 
were  made  by  different  members  of  the  party  during  the  two  following 
years,  but  these  were  simply  to  verify  previous  observations  or  to  correct 
apparently  conflicting  statements. 

This  monograph  is  purely  geological  in  its  scope  and  is  mainly  a 
careful  study  and  survey  of  a  comparatively  small  block  of  mountains, 
which  may  be  designated  the  Eureka  Mountains,  but  which  should  not  be 
confounded  with  the  Eureka  mining  district,  as  several  other  well  known  but 
less  important  mining  districts  also  lie  wholly  within  this  mountain  area. 

As  it  was  unmapped  and  only  occasionally  visited  by  geologists,  little 
had  been  accomplished,  except  for  the  immediate  purposes  of  mining,  toward 
investigating  its  structure  or  solving  its  many  geological  problems.  The 
Eureka  region  was  known  to  occupy  an  exceptionally  broad  expanse  of 
mountains,  affording  fine  geological  sections  if  carefully  worked  out,  and 
of  special  interest  for  the  purposes  of  comparative  study  in  other  regions  of 
the  Cordillera.  In  this  direction  scarcely  anything  had  been  accomplished. 

The  field  work,  as  planned,  could  not  have  been  completed  in  the 


Xil  PREFACE.      * 

allotted  time  except  for  the  untiring  energy  and  interest  of  all  those  connected 
with  the  survey.  In  the  geological  work  I  was  fortunate  in  having  the 
cooperation  of  two  thoroughly  equipped  assistants,  both  of  whom  have  since 
attained  honorable  distinction  by  published  writings  in  their  special  lines  of 
research.  To  Mr.  Charles  D.  Walcott  was  assigned  the  collection  of  the 
paleontological  material,  while  Mr.  Joseph  P.  Iddings  was  engaged  in  work- 
ing among  both  volcanic  and  sedimentary  rocks. 

The  report  appears  in  two  parts,  one  a  volume  of  text,  the  other  an 
accompanying  atlas  of  topographical  and  geological  maps  and  cross  sec- 
tions, and  as  the  text  is,  in  great  measure,  explanatory  of  the  atlas,  the  two 
can  be  considered  only  as  parts  of  the  same  work. 

A  paper  embodying  the  more  important  results  obtained  at  Eureka 
was  prepared  in  1882  and  published  in  the  Third  Annual  Report  of  the 
Survey  as  an  abstract  of  the  final  monograph.  It  was  accompanied  by 
a  geological  map  similar  to  sheet  iv  of  the  atlas.  The  volume  of  atlas 
plates  bears  the  imprint  of  1883,  but  is  now  issued  in  complete  form  for 
the  first  time.  In  its  more  essential  features  the  present  report  was  pre- 
pared several  years  ago,  but  the  completion  of  the  manuscript  has  been 
delayed  from  time  to  time  for  various  unforeseen  reasons,  mainly  by  press- 
ure of  other  duties.  It  presents,  as  concisely  as  is  consistent  with  clearness 
and  completeness,  the  principal  geological  facts  gathered  in  the  field  and 
such  general  deductions  as  have  been  drawn  from  their  study.  I  haye 
endeavored  to  make  each  chapter  complete  in  itself,  and  this  has  necessitated 
the  repetition  of  certain  observations,  as  a  large  number  of  facts  are  more  or 
less  related  to  the  subjects  discussed  in  the  different  chapters.  It  is  an 
advantage,  however,  to  the  special  reader,  to  have  such  facts  as  he  may 
need  brought  together  under  one  grouping,  and  not  to  feel  obliged  to 
search  through  the  volume  for  them. 

The  atlas  consists  of  thirteen  sheets.  The  preparation  of  the  topo- 
graphical map  was  intrusted  to  Mr.  F.  A.  Clark,  who  employed  three  able 
assistants  in  the  field — Mr.  G.  H.  Wilson,  assistant  topographer  with  the 
plane  table;  Mr.  Gr.  Olivio  Newman,  in  charge  of  triangulation,  and  Mr. 
Morris  Bien,  assistant  topographer. 

A  special  paper  by  Mr.  Iddings,  upon  the  microscopical  petrography  of 


I'liEFACE.  XIII 

the  eruptive  rocks  of  the  Eureka  District,  appears  as  an  appendix  to  this 
report.  It  presents  the  results  of  a  careful  examination  of  several  hundred 
thin  sections  prepared  from  a  large  number  of  rocks,  representing  every 
variety  known  to  occur  in  the  region.  It  is  a  concise  statement  of  results  of 
a  systematic  study  of  the  material  and  is  of  great  interest,  bearing  directly 
upon  many  geological  questions  connected  with  eruptive  masses.  Mr. 
Iddiiigs's  report  is  illustrated  by  six  plates,  four  of  which  are  reproductions 
of  photomicrographs,  showing  some  interesting  features  in  structure  of  fine 
groundrnass,  and  two  of  drawings  of  minute  crystals  and  microscopic  objects 
found  in  the  rocks.  At  the  time  these  photomicrographs  were  produced 
they  were  superior  to  anything  which  had  been  done  in  this  class  of  illus- 
tration. 

Mr.  Walcott's  report  upon  the  "Paleontology  of  the  Eureka  District" 
was  published  as  Monograph  VIII  of  the  U.  S.  Geological  Survey,  in  1884. 
It  gives  the  results  of  a  detailed  study  of  the  organic  forms  obtained 
throughout  a  wide  range  of  geological  formations,  the  region  having  proved 
an  exceptionally  rich  one  in  paleontological  material  from  Cambrian,  Devo- 
nian and  Carboniferous  rocks.  In  addition  to  the  descriptions  of  many 
forms  new  to  science  and  the  identification  of  over  five  hundred  species, 
the  report  contains  notes,  more  or  less  full,  upon  many  species  which 
presented  in  their  characters  or  geographical  distribution  information  not 
heretofore  published.  The  work  is  illustrated  by  over  five  hundred  and  fifty 
accurate  drawings  of  fossils,  arranged  on  twenty-four  plates.  Four  plates 
represent  the  fauna  of  the  Cambrian,  two  that  of  the  Silurian,  ten  that  of 
the  Devonian,  and  eight  that  of  the  Carboniferous.  All  specific  identifica- 
tions of  organic  forms  from  Eureka  referred  to  in  this  work  were  made 
by  Mr.  Walcott. 

After  the  completion  of  the  field  work  for  the  Eureka  map,  Mr.  J.  S. 
Curtis  began  his  investigations  of  the  ore  deposits  found  on  Ruby  Hill. 
The  surface  maps  published  by  Mr.  Curtis  were  taken  from  the  atlas  sheets 
accompanying  this  monograph.  Mr.  Curtis's  report  appeared  in  1884  as 
Monograph  VII  of  the  U.  S.  Geological  Survey,  and  is  entitled  "Silver-Lead 
Deposits  of  Eureka,  Nevada."  It  is  a  valuable  work  and  one  which  forms 
an  important  part  of  the  scientific  memoirs  relating  to  the  Eureka  District. 


XIV  PKEFACE. 

The  writer's  acknowledgments  are  due  to  many  gentlemen,  superin- 
tendents of  mines  and  others,  who  rendered  valuable  assistance  in  furnishing 
information  in  regard  to  the  country,  and  who  generously  afforded  every 
facility  in  the  prosecution  of  the  work.  Special  thanks  are  due  to  Mr.  R 
Kickard,  formerly  superintendent  of  the  Richmond  Mining  Company,  and 
to  Mr.  Thomas  J.  Read,  superintendent  of  the  Eureka  Consolidated  Mining 
Company. 

June  6,  1891.  ARNOLD  HAGUE. 


OUTLINE  OF  THIS  VOLUME. 


CHAPTER  I.  The  area  covered  by  the  geological  survey  of  the  Eureka  Mountains  embraces  a 
region  of  country  20  miles  square.  The  mountains  are  situated  on  the  Nevada  plateau  and  form  a 
somewhat  isolated  mass,  surrounded  on  all  sides  by  the  broad  detrital  valleys  so  characteristic  of 
the  Great  Basiu.  These  valleys  which  encircle  the  mountains  have  an  average  elevation  above  sea 
level  of  6,000  feet.  Rising  above  them  the  highest  peaks  attain  altitudes  varying  from  9,000  to  10,500 
feet.  In  strong  contrast  with  most  of  the  Great  Basin  ranges,  the  Eureka  Mountains  present  a  rough 
and  rugged  appearance,  with  varied  topographical  features.  • 

CHAPTER  II.  Sedimentary  rocks  belonging  either  to  the  Paleozoic  or  Quaternary  age  form  the 
greater  part  of  the  mountains  and  valleys.  Quaternary  beds  present  little  of  geological  interest, 
although  they  extend  over  wide  areas,  being  mainly  superficial  accumulations  composed  of  detrital 
material  brought  down  from  the  mountains  and  deposited  along  their  flanks  and  out  over  the  broad 
plains.  A  great  thickness  of  limestone,  sandstone,  and  shale,  which  make  up  the  Paleozoic  series  of 
rocks,  was  laid  down  under  varying  conditions  of  depth  of  water  and  rapidity  of  deposition  with 
only  one  well  recognized  unconformity  from  base  to  summit.  In  this  region  the  Paleozoic  age  was  a 
time  of  comparative  freedom  from  dynamic  movements.  Eureka  presents  no  direct  evidence  as  to 
the  time  mountain  building  took  place  other  than  that  the  region  was  elevated  into  a  broad  conti- 
nental land  mass  after  the  deposition  of  the  Upper  Coal-measure  limestone.  Reasons  are  assigned  for 
supposing  that  all  the  Great  Basin  ranges  owe  their  origin  to  a  post-Jurassic  movement.  The  folding, 
flexing,  and  faulting  which  outlined  the  mountains  broke  up  this  mass  of  sediments  into  six  sharply 
denned  orographic  blocks,  each  with  well  marked  structural  peculiarities.  These  mountain  blocks 
have  been  designated  as  follows:  Prospect  Ridge,  Fish  Creek  Mountains,  Silverado  and  County  Peak 
group,  Mahogany  Hills,  Diamond  Mountains,  and  Carbon  Ridge  and  Spring  Hill  group.  Taken 
together  these  six  blocks  present  a  compact  mass  of  mountains,  the  result  of  intense  lateral  com- 
pression and  longitudinal  strain.  Profound  longitudinal  faults  extend  the  entire  length  of  the  moun- 
tains, showing  a  displacement  of  beds  of  over  13,000  feet.  The  Paleozoic  sediments  measure  30,000 
feet  in  thickness,  with  Cambrian,  Silurian,  Devonian,  and  Carboniferous,  all  well  represented  by 
characteristic  fauna.  In  these  four  periods  fourteen  epochs  have  been  recognized. 

CHAPTER  III.  Cambrian  rocks  measure  7,700  feet,  divided  into  five  epochs,  as  follows:  Pros- 
pect Mountain  quartzite,  Prospect  Mountain  limestone,  Secret  Canyon  shale,  Hamburg  limestone,  and 
Hamburg  shale.  The  Middle,  Lower,  and  Upper  Cambrian  are  all  exposed.  On  the  crest  of  Prospect 
Ridge,  at  the  base  of  the  Cambrian  limestone,  occurs  the  Olenellus  shale,  the  oldest  fossiliferous  strata 
recognized  in  the  Great  Basin.  Hamburg  Ridge  carries  a  Potsdam  fauna  both  at  its  base  and  summit. 
Conformably  overlying  the  Cambrian  come  the  Silurian  rocks,  5,000  feet  in  thickness.  They 
fall  readily  into  three  epochs,  two  limestones  and  an  intervening  body  of  quartzite.  They  have 
been  designated  Pogonip  limestone,  Eureka  quartzite,  and  Lone  Mountain  limestone.  The  qnartzite 

XV 


4 

xvi  OUTLINE  OF  THIS  VOLUME. 

is  easily  distinguished  from  both  the  coareo  sands  and  grits  of  the  Cambrian  below  and  the  Carbon- 
iferous conglomerate  above.  An  unconformity  of  deposition  exists  between  the  Eureka  and  Lone 
Mountain  epochs.  Both  the  Trenton  and  Niagara  formations  are  included  within  the  Lone  Moun- 
tain epoch. 

CHAPTER  IV.  By  imperceptible  gradations  limestones  of  the  Lone  Mountain  epoch  pass 
upward  into  those  of  the  Devonian  period.  Devonian  rocks  occupy  a  larger  area  in  the  District  than 
those  of  any  other  period,  and  present  a  greater  thickness  than  either  the  Cambrian  or  Silurian. 
They  measure  8,000  feet,  divided  into  two  epochs :  A  bluish  limestone — the  Nevada  limestone — and  an 
argillaceous  black  shale — the  White  Pine  shale.  The  limestone  carries  a  rich  invertebrate  fauna  from 
base  to  summit.  The  black  shale  is  characterized  by  a  flora  which,  though  fragmentary,  is  suffi- 
ciently well  preserved  to  identify  the  genera  as  belonging  to  the  Upper  Devonian. 

The  Carboniferous  rocks  measure  9,300  feet,  which,  however,  does  not  quite  represent  their  full 
development,  the  uppermost  beds  having  undergone  more  or  less  erosion.  They  have  been  divided 
into  four  epochs,  as  follows :  Diamond  Peak  quartzite,  Lower  Coal-measure  limestone,  Weber  con- 
glomerate, aud  Upper  Coal-measure  limestone.  As  the  limestone  is  in  general  favorable  to  the  preser- 
vation of  organic  remains,  fossil-bearing  strata  occur  throughout  the  beds.  Three  salient  features 
mark  the  life  of  the  Lower  Coal-measures.  First,  the  occurrence  near  the  base  of  the  limestone  of  a 
fresh- water  fauna;  second,  the  varied  development  of  the  Lamellibranchiates  a  class  which  has  here- 
tofore been  but  sparingly  represented  in  the  collection  of  Carboniferous  fossils  from  the  Cordillera ; 
third,  the  mingling  near  the  base  of  the  horizon  of  Devonian,  Lower  Carboniferous,  aud  Coal-measure 
species  in  gray  limestone  directly  overlying  beds  characterized  by  a  purely  Coal-measure  fauna. 

In  the  first  range  to  the  east  of  the  Eureka  Mountains  Carboniferous  rocks  extend  for  miles 
along  the  edge  of  the  valley,  in  which  well  developed  coal  seams  occur. 

CHAPTER  V.  This  chapter  is  devoted  to  the  descriptive  geology  of  the  sedimentary  rocks. 
Each  orographic  block  is  described  in  detail,  beginning  with  Prospect  Ridge,  where  the  oldest  rocks 
occur,  followed  by  the  other  blocks  according  to  the  succession  of  strata.  It  gives  a  connected 
description  of  the  country  and  points  out  the  relations  of  the  different  mountain  masses  to  each  other. 

CHAPTER  VI.  A  discussion  of  the  Paleozoic  rocks  follows,  based  upon  the  facts  presented  in 
the  earlier  chapters.  It  is  shown  that  during  Paleozoic  time  a  pre-Cambrian  continent  existed  in 
western  Nevada  which  furnished  to  an  ocean  lying  to  the  eastward  an  enormous  amount  of  detrital 
material.  It  is  pointed  out  that  the  Eureka  region  was  situated  not  far  from  the  eastern  border  of  this 
land  mass,  and  that  a  large  part  of  its  coarse  conglomerates  aud  mechanical  sediments  must  have  been 
offshore  deposits.  .  The  geological  record  affords  proof  of  elevation  and  depression  th  roughont  Paleozoic 
time  with  intervals  of  shallow  water  and  proximity  of  land  areas  between  periods  of  relatively  deep 
seas.  Fresh-water  life,  plant  remains,  and  coal  seainb  at  different  horizons  furnish  additional  evidence 
of  shallow  water  and  offshore  deposits.  A  study  of  Paleozoic  rocks  in  other  parts  of  southern  and 
western  Nevada  exhibit  nearly  similar  geological  conditions  as  regards  sequence  of  beds.  This  is 
especially  well  shown  both  at  White  Pine  and  in  the  Highland  and  Pifion  ranges.  The  sequence  of 
strata,  both  to  the  north  and  south,  indicates  a  closer  agreement  with  the  conditions  of  sedimentation 
at  Eureka  than  the  many  exposures  situated  but  a  short  distance  eastward  of  the  latter  area.  The 
structural  relations  of  the,  different  orographic  blocks  to  each  other  and  the  outbursts  of  igneous  rocks 
are  well  brought  out  in  cross-section.  An  instructive  feature  at  Eureka  is  the  close  relationship 
between  the  anticlinal  and  synclinal  folds  to  the  profound  north  and  south  faults. 

CHAPTER  VII.  Pre-Tertiary  igneous  rocks  play  a  very  subordinate  part.  They  may  be  classed 
under  three  heads:  Granite,  granite-porphyry,  and  quartz-porphyry.  The  granite  occupies  a  limited 
area  on  Prospect  Ridge.  Both  the  granite  and  quartz-porphyries  occur  as  dikes.  Structural  varia- 
tions in  the  dikes  are  mainly  dependent  upon  the  chilling  effect  of  cold  contact  walls  upon  a  rapidly 


OUTLINE  OF  THIS  VOLUME.  XVII 

cooling  molten  mass.  The  width  of  the  (like  has  much  to  do  in  determining  the  physical  conditions 
governing  crystallization.  A8  regards  the  age  of  the  dikes  little  is  known  other  than  that  they  pene- 
trate Siluriaii  strata. 

CHAPTER  VIII.  The  Eureka  District  offers  no  direct  proof  of  the  age  or  duration  of  volcanic 
energy,  although  evidence  based  upon  observations  elsewhere  in  the  Great  Basin  points  to  the 
conclusion  that  the  lavas  belong  to  the  Tertiary  period,  and  probably  the  greater  part  of  them 
to  the  Pliocene  epoch.  They  broke  out  in  four  ways:  First,  through  profound  fissures  along 
meridional  lines  of  displacement;  second,  following  lines  of  orographic  fracture,  they  border 
and  encircle  large  uplifted  masses  of  sedimentary  strata ;  third,  they  occur  as  dikes  penetrating  the 
sedimentary  rocks;  fourth,  they  occur  in  one  or  two  relatively  large  bodies,  notably  Richmond  Moun- 
tain and  Pinto  Peak,  along  lines  of  displacement.  The  sequence  of  lavas  was  hornblende-andesite, 
hornblende-mica-andesite,  dacite,  rhyolite,  pyroxene-andesite,  and  basalt.  The  lavas  display  a  great 
variety  of  volcanic  products  in  both  chemical  and  mineral  composition.  They  are  all  derived  from  a 
common  source,  a  homogeneous  molten  mass.  They  are  due  to  a  process  of  differentiation  by  molec- 
ular change  within  the  molten  mass  under  varying  conditions  of  pressure  and  temperature.  Starting 
with  a  magma  of  intermediate  composition,  the  extreme  products  of  such  a  differentiation  are  rhyolite 
and  basalt. 

CHAPTER  IX.  In  the  Eureka  District  the  ores  occur  in  sedimentary  rocks  belonging  to  the 
Cambrian,  Silurian,  and  Devonian  periods,  and  may  be  found  in  all  horizons,  except  the  Secret 
Canyon  and  Hamburg  shale,  from  the  base  of  the  Prospect  Mountain  limestone  to  the  summit  of  the 
Nevada  limestone.  Through  17,000  feet  of  strata  ores  have  been  deposited  in  sufficiently  large 
bodies  to  encourage  mining  exploration.  The  most  productive  deposits  have  been  found  in  Cambrian 
rocks,  but  this  is  owing  to  orographic  and  structural  conditions  rather  than  the  geological  age  of 
strata  or  chemical  nature  of  sediments.  Nearly  all  the  more  productive  mines  are  included  within 
the  beds  which  form  the  Prospect  Mountain  uplift  between  the  Hoosac  and  Spring  Valley  faults.  The 
ore  followed  the  rhyolite  and  is  consequently  Pliocene  or  post-Pliocene  age.  All  the  ores  came  from 
below  and  were  originally  deposited  as  sulphides.  They  were  subsequently  oxidized  by  atmospheric 
agencies,  mainly  surface  waters  percolating  through  the  rocks. 

In  Appendix  A,  Mr.  C.  D.  Walcott  gives  a  systematic  list  of  fossils  from  each  formation  found 
at  Eureka. 

In  Appendix  B,  Mr.  Joseph  P.  Iddings  discusses  the  microscopical  petrography  of  the  crystal- 
line rocks.     It  is  a  thorough  study  of  the  mineral  and  structural  character  of  the  rocks  and  is  illus- 
trated by  several  plates. 
MON  XX II 


GEOLOGY  OF  THE  EUREKA  DISTRICT. 


BY  ARNOLD  HAGUE. 


CHAPTER   I. 

GENERAL  DESCRIPTION. 

The  Eureka  District  is  situated  on  the  Nevada  plateau  in  the  central 
part  of  the  state  of  Nevada,  midway  between  the  basin  of  Lake  Lahontan 
westward  and  the  basin  of  Lake  Bonneville  eastward.  The  area  covered 
by  the  geological  and  topographical  survey  embraces  a  region  of  country 
20  miles  square,  lying  partly  in  the  county  of  Eureka  and  partly  in  the 
county  of  White  Pine. 

The  meridian  of  116°  west  from  Greenwich  passes  just  westward  of 
the  center  of  the  examined  area,  and  the  39°  30'  parallel  of  north  latitude 
crosses  Ruby  Hill,  the  seat  of  the  present  activity  in  precious-metal 
mining. 

Nevada  plateau.— On  the  Nevada  plateau  the  broad  central  north  and 
south  valleys,  lying  between  meridional  mountain  ranges,  reach  an  aver- 
age altitude  of  6,000  feet  above  sea-level,  the  country  falling  away  grad- 
ually on  both  sides  till  at  Salt  Lake,  in  Utah,  the  altitude  is  4,250  feet,  and 
at  Carson  and  Humboldt  Lakes,  in  Nevada,  3,800  feet  above  sea  level. 
These  valleys,  however,  compared  with  those  of  the  depressed  areas  adjoin- 
ing the  plateau,  are  relatively  narrow,  with  few  marked  exceptions,  seldom 
measuring  more  than  10  or  12  miles  in  width.  In  general  the  broader 
physical  features  of  the  Great  Basin  ranges  are  much  the  same  all  the  way 
MON  xx 1  1 


2  <1  EULOGY  OF  THE  EUKEKA  DISTRICT. 

from  the  bold  escarpment  of  the  Sierra  Nevada  of  California  to  the  precip- 
itous wall  of  the  Wasatch  Mountains  of  Utah,  the  distance  across  the 
widest  part  in  an  east  and  west  line  being  about  425  miles.  These  ranges 
form  long,  narrow  mountain  uplifts  with  sharply  defined  limits,  rising  with 
more  or  less  abruptness  above  dreary  intervals  of  desert.  Their  nearly 
uniform  trend  and  the  remarkable  parallelism  of  the  lines  of  upheaval  of  the 
older  sedimentary  ridges  present  the  most  marked  feature  of  the  region. 
In  width  they  seldom  exceed  8  miles,  but  frequently  extend  in  an  unbroken 
line  for  more  than  100  miles  in  length,  with  serrated  peaks  and  ridges  rising 
from  2,000  to  6,000  feet  above  adjacent  valleys.  For  the  most  part  they 
possess  a  simple  topographical  structure  and  a  simple  drainage  system. 
They  are  characterized,  more  especially  the  lower  ranges,  by  absence  of 
trees,  and  in  many  cases  are  nearly  bare  of  all  vegetation,  presenting  rough, 
rugged  slopes  of  naked  rock. 

On  the  higher  parts  of  the  plateau  the  ranges,  reaching  a  greater  alti- 
tude, partake  more  of  an  Alpine  or  sub-Alpine  character.  Precipitation 
of  moisture  is  more  abundant,  as  seen  both  in  the  more  frequent  rains  of 
slimmer  and  snows  of  winter.  A  greater  precipitation  produces  larger  and 
more  frequent  streams,  and  a  continued  moisture  favors  a  varied  vegeta- 
tion— the  spurs  and  ridges  being  more  or  less  covered  with  a  dwarfed  and 
stunted  forest  growth,  and  the  long  slopes  with  nutritious  grasses. 

These  salient  features  distinguish  the  ranges  of  the  Nevada  plateau 
from  those  of  Lake  Lahontan  and  Lake  Bonneville  Basins,  which  present  a 
more  arid  and  desolate  aspect.  A  striking  feature  of  nearly  all  these  ranges 
is  their  isolated  position,  only  a  few  of  them  presenting  outlying  spurs  or 
low  lines  of  rolling  foothills.  Occasionally  inferior  ridges  of  sedimentary 
beds  stretch  diagonally  across  valleys  from  one  range  to  another,  com- 
pletely shutting  in  the  intermediate  valley,  and  still  more  frequently  out- 
bursts of  volcanic  rocks  in  irregular  flows  serve  to  unite  in  confused  masses 
bodies  of  sedimentary  formations  otherwise  distinct. 

Midway  between  the  Sierra  and  the  Wasatch  stand  the  East  llum- 
boldt  Mountains,  the  most  prominent  range  in  the  Great  Basin.  They  lire- 
sent,  not  only  by  reason  of  the  greater  number  of  rugged  and  commanding 
peaks,  many  of  them  attaining  an  elevation  over  11,000  feet  above  sea 


EUREKA   MOUNTAINS.  3 

level,  but  by  their  broad,  massive  proportions,  long,  unbroken  ridges,  and 
Alpine  character,  the  boldest  uplift  on  the  Nevada  plateau.  Next  west 
from  the  Humboldt  occurs  the  Diamond  Range,  followed  by  the  Pinon 
Range,  with  the  broad  Diamond  Valley  lying  between  them.  Southward 
the  southern  extremities  of  these  two  ranges  enter  the  Eureka  District  and 
form  a  part  of  its  mountainous  region. 

On  the  plateau,  among  the  more  marked  exceptions  to  the  long  narrow 
ranges  which  rib  the  surface  of  the  country,  may  be  mentioned  the  Rob- 
erts Peak  Group,  connecting  the  Wahweah  with  the  Pinon  Range,  the 
White  Pine  Mountains,  and  the  subject  of  the  present  report,  the  moun- 
tains of  the  Eureka  District. 

Eureka  Mountains.— The  Eureka  District  forms  a  rough  mountain  block 
standing  out  prominently  by  itself,  except  for  its  narrow  connections  with 
both  the  Pinon  and  Diamond  Ranges,  almost  as  completely  isolated  from  its 
neighbors  as  the  longer  parallel  ranges.  As  a  mountain  mass,  however, 
although  well  deserving  such  a  distinction,  it  has  never  received  any  definite 
appellation  which  would  include  all  its  members,  it  being  made  up  of  por- 
tions of  several  ranges  and  short  uplifted  blocks  sp  intimately  connected 
and  inosculated  as  to  form  both  topographically  and  geologically  a  single 
group,  hemmed  in  on  all  sides  by  the  characteristic  detrital  valleys.  To 
the  north  Diamond  Valley,  which  may  be  taken  as  a  type  of  the  higher 
valleys  of  the  Great  Basin,  extends  for  over  40  miles  in  an  unbroken  plain, 
the  lowest  part  of  the  depression  being  covered  in  winter  by  a  broad,  shal- 
low sheet  of  water,  which,  upon  evaporation,  presents  during  the  greater 
part  of  the  year  a  hard,  level  floor,  strongly  impregnated  with  salt.  Con- 
siderable quantities  of  salt  for  metallurgical  purposes  have  been  collected 
from  the  shores  of  the  small  lakes  at  the  northern  end  of  the  valley.  To 
the  south  of  the  district  lies  the  broad  basin  of  Fish  Creek  Valley,  con- 
necting with  Newark  Valley  on  the  east  side  of  Diamond  Range,  while 
the  Antelope  Valley  cuts  off"  the  Eureka  District  on  the  west  side  from  the 
neighboring  mountains.  All  these  valleys  stand  at  about  the  same  elevation 
above  sea  level,  and  offer  to  the  eye  a  monotonous  olive-gray  color  derived 
from  a  vigorous  growth  of  the  Artemcsia  tridenlata  which  covers  all  the  low- 
lands except  the  central  portions  of  the  broader  basins. 


4  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

It  is  doubtful  if  any  urea  of  equal  extent  iu  Nevada  possesses  more 
varied  physical  features  with  such  strongly  marked  contrasts  than  the  Eu- 
reka District.  In  close  proximity  may  be  seen  long  serrated  ridges,  broad 
summits,  gently  inclined  tables  of  nearly  horizontal  sedimentary  beds,  with 
abrupt  escarpments  along  canyon  walls,  and  highly  tilted  strata  in  rough 
irregular  spurs.  And,  as  might  be  expected  in  a  country  made  up  of  indi- 
vidual blocks  and  parts  of  ranges  and  so  interlocked  as  to  form  one  broad 
mass,  the  region  is  characterized  by  broad  shallow  basins,  long  narrow 
ravines,  and  winding  valleys,  presenting  a  more  than  ordinarily  accidented 
surface  with  an  intricate  structure.  Above  the  broad  base  of  the  surround- 
ing sage-brush  valleys  rise  many  prominent  peaks  from  2,500  to  4,500  feet. 
Diamond  Peak,  in  the  northeast  corner  of  the  district,  at  the  southern  ex- 
tremity of  Diamond  Range,  is  the  culminating  point,  measuring  10,637  feet 
above  sea  level,  and,  with  the  exception  of  the  high  summits  in  the  East 
Humboldt  Range,  is  one  of  the  loftiest  peaks  on  the  Nevada  plateau. 
Prospect  Peak,  on  the  central  ridge,  and  the  second  point  in  the  district, 
measures  9,604  feet,  while  Atrypa  Peak,  to  the  southwest  on  the  same 
ridge,  has  an  altitude  of  9,063  feet  above  sea  level.  Other  points  are  White 
Cloud  Peak,  the  highest  point  on  a  broad  plateau-like  ridge,  8,950  feet; 
Alpha  Peak,  8,985  feet;  and  Woodpecker's  Peak,  8,598  feet;  all  of  them 
being  formed  of  sedimentary  rocks.  Among  volcanic  mountains  may  be 
mentioned  Richmond  Mountain,  just  east  of  the  town  of  Eureka,  which  rises 
to  a  height  of  8,392  feet,  and  Pinto  Peak,  an  isolated  cone  in  the  center  of 
the  district,  reaches  an  altitude  of  7,880  feet  above  sea  level. 

Up  to  the  time  of  the  rapid  development  of  the  mining  interests  upon 
Ruby  Hill  and  Prospect  Mountain,  the  slopes  and  ridges  about  Eureka 
were  exceptionally  well  supplied  with  an  arborescent  growth,  a  condition 
which  was  due  partly  to  the  number  of  high  peaks  but  in  great  part  to 
broad  masses  of  mountains  acting  as  condensers  of  desert  moisture.  To- 
day, so  great  has  been  the  demand  for  wood  B,nd  charcoal  in  the  reduction 
of  lead  ores,  that  the  mountains  are  as  bare  of  trees  as  any  part  of  the 
Great  Basin.  Several  species  of  pines,  dwarfed  junipers  (Juniperus  occiden- 
talis),  and  mountain  mahogany  (Cercocarpus  Icedifoliits),  which  attains  a 
height  of  over  20  feet,  are,  or  rather  were,  the  prevailing  trees,  but  are  now 


SOIL— CLIMATE.  5 

found  only  in  a  few  areas  preserved  by  their  owners  for  future  use,  at  no 
distant  day.  Not  only  have  the  Eureka  Mountains  lost  their  forests,  but 
the  neighboring  mountains  for  long  distances  have  been  devastated  to  fur- 
nish fuel  for  the  smelting  furnaces.  Some  idea  may  be  obtained  of  the 
enormous  consumption  of  wood  from  the  statement  that  10,000  bushels  of 
charcoal  are  required  daily  for  the  smelting  furnaces  when  the  works  are 
running  their  usual  force,  and  that  for  five  or  six  years  the  daily  consump- 
tion was  rather  over  than  under  that  amount. 

Soil.— Nature  presents  a  barren,  arid  appearance.  Perennial  streams  in 
the  ravines  are  exceptional,  other  than  those  found  on  the  slopes  of  Diamond 
Peak.  Fresh  water  springs  lie  scattered  about  the  mountains  and  fur- 
nish a  scanty  supply  of  water,  barely  sufficient  to  meet  the  wants  of  the 
people.  A  few  deep  wells  have  been  successfully  sunk  in  the  broader 
valleys.  Vegetation  is  everywhere  limited,  and  is  mainly  confined  to  bunch 
grasses  on  the  mountain  slopes  and  sage  brush  in  the  open  valleys. 

As  the  valleys  are  mainly  filled  with  coarse  detrital  material  from 
mountain  slopes,  soils  suitable  for  agricultural  purposes  occupy  very 
small  areas,  and  are  found  only  in  the  broader  basins.  In  the  favored 
spots  where  water  for  irrigation  purposes  can  be  readily  obtained,  all  the 
more  hardy  vegetables  grow  well,  and  are  of  excellent  quality,  but  nearly 
all  crops  suffer  from  early  frosts.  In  no  sense  can  the  country  be  regarded 
as  an  agricultural  one,  and  cultivation  of  the  soil  is  remunerative  to 
the  farmer  only  by  reason  of  the  very  high  prices  received  for  his  produce. 

climate.— A  rigorous  winter,  a  long  hot  summer,  a  dry  atmosphere,  with 
a  light  precipitation  of  moisture,  are  characteristic  climatic  features  of  the 
Eureka  District.  In  summer,  rainfalls  are  limited  to  showers,  frequently 
very  severe,  but  of  short  duration,  and  what  are  commonly  known  as  cloud- 
bursts are  by  no  means  uncommon  during  late  July  and  early  August. 
The  clouds,  late  in  the  afternoon,  centering  over  Prospect  Peak,  break  with 
such  force  that  many  people  caught  without  warning  have  been  drowned. 
In  July,  1874,  a  severe  storm  and  flood  destroyed  seventeen  lives,  and 
carried  off  property  to  the  value  of  many  thousands  of  dollars. 

During  the  period  of  our  survey  careful  meteorological  observations 
were  made  throughout  the  summer.  Snow  fell  in  the  month  of  May  no 


6  GEOLOGY  OF  THE  EUEEKA  DISTRICT. 

less  than  eight  times,  and  again  on  June  10  and  11  In  summer  the  days 
are  warm,  and  for  the  most  part  cloudless;  the  nights  cool.  The  daily 
variation  between  the  maximum  and  minimum  thermometers  was  always 
very  considerable,  frequently  showing  a  difference  of  40°  F.  For  the  three 
summer  months  of  June,  July,  and  August,  of  1880,  the  maximum  ther- 
mometer in  the  shade  stood  over  90°  F.  on  eighteen  days,  or  one  day  in 
six.  As  the  climate  is  very  dry,  the  heat  was  seldom  oppressive,  except 
in  some  inclosed  basin  or  valley.  As  early  as  August  30,  the  thermometer 
fell  below  the  freezing  point,  and  on  October  9  a  light  fall  of  snow  covered 
both  mountain  and  valley. 

History.— In  the  summer  of  1864  the  first  locations  of  mining  property 
were  made  in  New  York  Canyon,  on  the  easteni  side  of  Prospect  Mountain, 
near  the  present  "  76  "  Mine.  This  property  was  known  as  the  Eureka 
Mine,  and  although  it  never  fulfilled  the  expectations  of  its  original  owners, 
it  transferred  its  name  to  the  very  successful  property  on  Ruby  Hill  and 
subsequently  gave  a  name  to  the  town,  to  the  mining  district,  to  the  county, 
and  finally  to  the  neighboring  group  of  mountains.  The  original  property 
gave  so  little  promise  that  the  district  was  finally  abandoned.  In  mining 
operations  very  little  was  accomplished  until  the  spring  of  1869,  when  im- 
portant discoveries  were  made  on  Ruby  Hill  and  active,  intelligent  work 
was  undertaken.  The  Champion  and  Buckeye  claims  on  the  south  side  of 
Ruby  Hill  were  the  first  properties  located,  and  soon  afterward  the  ground 
was  broken  on  the  now  famous  Richmond  and  Tip  Top  Mines.  From  that 
time  forward  mining  operations  on  Ruby  Hill  have  gone  on  steadily,  and 
to-day  the  Eureka  District  is  the  most  successful  mining  region  in  the  state 
of  Nevada.  Success  on  Ruby  Hill  was  quickly  followed  by  active  enter- 
prise developing  mining  locations  on  both  slopes  of  the  ridge  of  Prospect 
Mountain,  in  Secret  Canyon,  and  in  the  Silverado  Hills  in  the  southwest 
corner  of  the  district. 

Estimates  of  the  value  of  the  ore  production  of  the  district  since  the 
first  shipment  of  crude  bullion  in  1869  are  as  follows : 

From  1869  to  1873 $10,000,000 

From  1873  to  January,  1883 50,000,000 

Total 60,000,000 


H1STOKY  OF  THE  DISTRICT.  7 

One-third  of  this  amount,  according  to  the  best  estimates,  was  gold, 
and  two-thirds  silver.  The  product  in  lead  is  not  so  easily  determined,  but 
it  is  not  far  from  225,000  tons,  an  amount  sufficient  to  affect  the  market 
price  of  lead  in  all  the  great  commercial  centers  of  the  world. 

Around  this  industry  has  grown  up  the  town  of  Eureka,  which  is  the 
center  of  population  and  trade  for  this  part  of  the  state.  It  is  a  long,  narrow 
settlement,  lying  in  the  main  northern  drainage  channel  of  the  mountains, 
and  sheltered  on  the  east  side  by  Richmond  Mountain.  Here  are  located 
the  smelting  furnaces  of  both  the  large  companies. 

The  Eureka  and  Palisade  Railway,  88  miles  in  length,  connects  the 
town  with  the  Central  Pacific  Road  at  Palisade.  Branch  tracks  connect 
with  the  Eureka  Consolidated  and  Richmond  furnaces,  the  former  at  the 
lower,  and  the  latter  at  the  upper  end  of  the  town,  and  these  again  by 
a  somewhat  sinuous  course  with  the  principal  mines,  which  are  situated 
about  two  and  one-half  miles  southwest  of  Eureka.  There  are  an  imposing, 
well  built  court  house,  three  or  four  churches,  and  several  blocks  of  brick 
stores  and  warehouses  in  the  town.  It  supports  two  daily  papers,  which 
have  a  considerable  influence  and  a  wide  circulation  throughout  the  state. 

Ruby  Hill,  the  only  other  town  of  any  importance  in  the  district,  is  a 
flourishing  place,  nearly  the  entire  population  being  actively  engaged  in 
mining  in  the  immediate  neighborhood.  It  is  built  on  the  north  and  east 
sides  of  an  isolated  hill  which  bears  the  same  name,  and  on  which  are 
located  all  the  more  prominent  mines,  including  the  Albion,  Richmond, 
Eureka  Consolidated,  Phoenix,  and  Jackson  properties.  On  the  slopes  to 
the  north  are  situated  the  Bullwhacker  and  Williamsburg  mines,  while  to 
the  southward  of  Ruby  Hill,  on  Prospect  Ridge,  are  found  the  Dunderberg 
and  Hamburg  properties  and  others  of  more  or  less  importance. 


CHAPTER    II. 

GEOLOGICAL  SKETCH  OF  THE  EUREKA  DISTRICT. 

Sedimentary  rocks,  belonging  either  to  the  Paleozoic  or  Quaternary 
period,  form  by  far  the  greater  part  of  the  mountains  and  valleys  of  the 
Eureka  District.  The  beds  of  the  Quaternary  present  but  little  of  geological 
interest,  and  although  they  extend  over  wide  areas  they  are,  in  most 
instances,  superficial  accumulations  composed  of  detrital  material  brought 
down  from  the  mountains  and  deposited  along  their  flanks,  concealing  the 
underlying  rocks  of  the  foothills.  Igneous  rocks  play  a  most  important 
part  in  the  geological  history  of  the  region,  but  nevertheless  do  not  form  an 
imposing  feature  of  the  individual  mountain  uplifts,  appearing  either  as  ex- 
travasated  masses  along  lines  of  faulting,  or  as  larger  bodies  encircling  and 
lying  outside  the  main  blocks  of  sedimentary  formations.  The  older  crys- 
talline rocks  offer  a  still  less  marked  topographical  feature  of  the  country, 
occupying  very  limited  areas  in  the  older  Paleozoic  limestones,  where  they 
appear  as  intruded  masses  exposed  by  erosion. 

It  is  doubtful  if  within  the  province  of  the  Great  Basin  there  can  be 
found  any  region  of  equally  restricted  area  surpassing  the  Eureka  District 
in  its  grand  exposures  of  Paleozoic  formations,  especially  of  the  lower  and 
middle  portions. 

The  great  thickness  of  limestone  aiid  sandstone  of  which  the  Paleozoic 
is  composed  was  laid  down  under  varying  conditions  of  depth  of  water  and 
rapidity  of  deposition,  with  only  one  well  recognized  unconformity  from  its 
base  to  summit.  In  this  region  the  Paleozoic  age  was  a  time  of  compara- 
tive freedom  from  dynamic  movements.  Most  geologists  who  have  given 
any  attention  to  the  history  of  the  Great  Basin  ranges  substantially  agree 
that  the  movements  that  finally  built  up  the  mountains  began  after  the 
close  of  Paleozoic  time,  and  that  between  the  Carboniferous  and  the  close 

8 


AGE  OF  MOUNTAIN  BUILDING.  9 

of  the  Jurassic  period  took  place  the  folding,  flexing  and  faulting  of  the 
beds  which  outlined  the  structural  features  of  nearly  all  the  meridional 
ranges  between  the  abrupt  walls  of  the  Wasatch  and  those  of  the  Sierra 
Nevada.  At  Eureka  no  direct  evidence  is  offered  as  to  the  time  when  this 
mountain  building  took  place  other  than  that  the  region  was  finally  lifted 
above  the  ocean  after  the  deposition  of  the  Upper  Coal-measures.  So  far 
as  the  mountains  themselves  are  concerned,  there  is  a  total  lack  of  evidence 
that  the  blocking  out  of  the  ridges  did  not  begin  at  the  close  of  the  Paleozoic 
period,  but,  on  the  other  hand,  all  observations  tend  to  show  that  whenever 
and  by  whatever  causes  the  other  Great  Basin  ranges  were  uplifted,  the 
same  orographic  conditions  which  prevailed  elsewhere  held  true  for  the 
Eureka  Mountains.  In  other  words,  the  Eureka  Mountains  were  a  part  of 
a  more  extended  geological  province. 

According  to  the  conclusions  of  Mr.  Clarence  King,1  based  upon  the 
observations  of  the  geologists  of  the  Fortieth  Parallel  Exploration,  the 
mountains  west  of  the  Havallah  Range  and  the  meridian  of  117°  30'  belong 
to  a  post-Jurassic  upheaval,  and  to  the  west  of  this  line  there  existed  during 
Paleozoic  time  an  elevated  continental  area  which  fumished  the  material 
accumulated  in  an  ocean  basin  to  the  east.  At  the  close  of  the  Paleozoic 
this  oceanic  area,  stretching  as  far  eastward  as  the  Wasatch,  was  lifted  up 
into  a  broad  laud-mass,  and  the  former  continental  region  sank  below  the 
water  and  in  turn  became  an  ocean  basin.  From  the  Wasatch  westward 
to  this  ancient  shore  line  the  mountain  ridges  exhibit  much  in  common  in 
their  structural  and  physical  features,  being  made  up  in  great  measure  of 
Paleozoic  strata,  whereas  from  this  boundary  westward  the  ranges  show 
a  marked  contrast  in  the  nature  of  their  sedimentation  and  bear  ample 
paleontological  evidence  of  their  Mesozoic  age.  Over  this  latter  area,  not- 
ably in  the  West  Humboldt,  Piute,  and  Augusta  Mountains,  limestones 
characteristic  of  the  Triassic  and  Jurassic  have  been  described  in  detail  by 
the  geologists  of  the  Fortieth  Parallel  Exploration,2  while  to  the  east  of  this 
shore  line  no  Mesozoic  rocks  occur.  Mr.  King  assigns  excellent  reasons  for 


1  Geological  Exploration  of  the  Fortieth  Parallel,  vol.  i,  Systematic  Geology,  p.  733.     Washing- 
ton: 1878. 

"Geological   Exploration  of  the   Fortieth    Parallel,   vol.   n.   Descriptive  Geology,  pp.  657,  711, 
ami  724.     Washington.  1877. 


10  GEOLOGY  OF  THE  EUEEKA  DISTRICT. 

the  opinion  that  all  the  Great  Basin  ranges  across  Utah  and  Nevada  were 
uplifted  at  the  same  time  under  identical  dynamic  influences,  and  conse- 
quently owe  their  origin  mainly  to  a  post-Jurassic  movement. 

This  indicates  a  marked  unconformity  between  the  Carboniferous  and 
Triassic,  but  it  neither  necessitates  nor  precludes  the  beginning  of  mountain 
building  over  the  Paleozoic  area  at  the  time  of  the  uplifting  of  the  conti- 
nental laud-mass  from  beneath  the  ocean.  Nowhere  throughout  this  region, 
any  more  than  at  Eureka,  have  the  Great  Basin  ranges  as  yet  offered  any 
direct  evidence  of  folding  accompanying  this  elevation,  yet  it  would  seem 
highly  probable  that  some  crumpling  of  strata  might  have  taken  place  before 
the  main  blocking  out  of  the  mountain  ridges  at  the  close  of  Jurassic  time. 

Most  of  the  Great  Basin  ranges  are  narrow,  longitudinal  ridges,  and 
while  they  present  much  in  common  as  to  their  origin  and  primary  struc- 
ture, each  possesses  its  own  special  physical  features  due  to  local  dynamic 
conditions.  Most  of  them  are  formed  by  direct  lateral  compression  result- 
ing in  anticlinal  folds,  occasionally  accompanied  by  synclines.  Some  of 
them  are  simple  mouoclinal  ridges,  representing  one  side  of  an  anticlinal 
axis.  Still  others  exhibit  great  complexity  of  structure  with  both  folding 
and  faulting  along  the  meridional  axes  of  the  ranges,  with  which  are  asso- 
ciated transverse  faults  and  folds  striking  obliquely  across  the  topograph- 
ical trend  of  the  uplifted  mass. 

Orographic  Blocks.— The  Eureka  Mountains  lie  near  the  western  edge  of 
what  was  at  one  time  the  Paleozoic  ocean.  The  nearness  of  these  uplifted 
beds  to  an  older  pre-Paleozoic  continent  is  in  some  measure  indicated  by  the 
relatively  great  amount  of  disturbance  of  strata  and  plication  of  mountain 
masses  as  compared  with  the  more  gently  inclined  strata,  and  simplicity  of 
structure  found  farther  to  the  eastward.  Unlike  the  ordinary  type  of  nar- 
row ridges,  the  Eureka  Mountains  exhibit  a  solid  mountain  mass  over  20 
miles  in  width,  including  several  uplifted  blocks  whose  length  does  not 
greatly  exceed  their  width.  Taken  together  they  present  a  compact  mass 
of  mountains  thrown  up  by  intense  lateral  compression  accompanied  by 
longitudinal  strain.  The  forces  which  brought  about  the  elevation  of  the 
mountains  produced  an  intricate  structure  with  powerful  flexures  and  folds 
and  broke  up  this  immense  thickness  of  sediments  into  individual  blocks 


PALEOZOIC  SECTION.  11 

accompanied  by  profound  longitudinal  faults,  several  of  which  extend  the 
entire  length  of  the  mountains,  and  have  played  a  most  important  part  in 
bringing  about  the  present  orographic  conditions. 

Although  these  mountain  masses  stand  so  intimately  related  to  each 
other  that  it  is  frequently  difficult  to  draw  sharp  topographical  lines  between 
them,  the  Eureka  Mountains  may  be  divided  into  six  blocks  with  well 
marked  structural  and  geological  differences.  These  blocks  may  be  desig- 
nated as  follows: 

Prospect  liidge. 

Fish  Creek  Mountains. 

Silverado  and  County  Peak  group. 

Mahogany  Hills. 

Diamond  Mountain. 

Carbon  Ridge  and  Spring  Hill  group. 

Paleozoic  Section.— As  already  mentioned,  the  Eureka  Mountains  lie  just 
eastward  of  the  old  shore  line.  In  this  and  the  following  chapters  the 
evidence  is  presented,  derived  from  the  history  of  the  rocks  themselves,  to 
show  the  close  proximity  of  a  land  area  when  the  beds  were  laid  down. 
The  nature  of  these  off-shore  deposits  near  the  western  border  of  an  old 
Paleozoic  sea  form  one  of  the  principal  objects  of  this  investigation.  Much 
of  the  material,  such  as  the  coarser  conglomerates,  must  necessarily  have  been 
off-shore  deposits  The  sedimentary  rocks  which  make  up  the  mountains 
present  a  great  development  of  limestones,  quartzites,  sandstones,  and  shales, 
comprising  many  thousands  of  feet  of  Cambrian,  Silurian,  Devonian,  and 
Carboniferous  beds.  From  the  lowest  exposed  members  of  Cambrian  strata 
to  the  top  of  the  Coal-measures  there  are  represented  a  series  of  sedimentary 
deposits  30,000  feet  in  thickness.  Nowhere  within  the  limits  of  the  Eureka 
district  can  there  be  found  any  one  exposure  which  shows  the  beds  with- 
out a  break  in  their  continuity,  the  longest  unbroken  section  representing 
about  one-third  of  the  entire  sequence  of  strata,  yet  the  region  offers  in  so 
many  instances  such  continuous  exposures  of  beds  and  so  many  in  which 
the  series  of  strata  overlap  each  other  with  such  a  constant  repetition  of 
beds,  that  the  reconstruction  of  the  entire  section  is  easily  made  out  when 
the  individual  parts  are  carefully  compared  and  studied.  The  reason  why 
there  is  no  one  unbroken  section  may  be  readily  understood  by  a  glance 


12  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

at  the  map  which  shows  how  the  sedimentary  strata  have  been  broken  up 
into  separate  mountain  blocks,  each  made  up  of  a  portion  of  the  entire 
thickness  of  beds. 

In  the  four  grand  periods  of  Paleozoic  time  represented  at  Eureka,  14 
epochs  have  been  recognized  :  5  in  the  Cambrian,  3  in  the  Silurian,  2  in  the 
Devonian,  and  4  in  the  Carboniferous. 

With  a  single  exception  local  geographical  names  have  been  employed 
to  designate  the  different  epochs  into  which  the  Cambrian,  Silurian,  and 
Devonian  have  been  divided.  Heretofore,  throughout  the  Great  Basin  the 
division  of  the  larger  periods  into  epochs  has  not  been  deemed  necessary, 
the  individual  horizons  not  having  been  studied  sufficiently  in  detail  to 
require  it.  The  exception  is  made  in  favor  of  the  Pogonip  limestone,  a 
name  first  applied  by  the  Geological  Exploration  of  the  Fortieth  Parallel  to 
the  belt  of  limestone  which  forms  the  base  of  the  Silurian.  In  the  Carbon- 
iferous period  a  large  quartzite  body  at  the  base  of  the  series  has  been 
designated  the  Diamond  Peak  quartzite,  but  for  the  remaining  epochs  the 
well  known  names  Lower  Coal-measures,  Weber  conglomerates,  and  Upper 
Coal-measures  are  retained,  notwithstanding  some  serious  objection  to  the 
use  of  the  term  Coal-measures  in  this  region. 

Each  of  the  six  blocks  expose  several  thousand  feet  of  strata,  and 
while  they  frequently  overlap  each  other  no  two  of  them  represent  precisely 
the  same  horizons,  although  the  Diamond  Range  includes  within  its  strata 
the  beds  which  make  up  the  Carbon  Ridge  and  Spring  Hill  blocks.  The 
six  blocks  essentially  correspond  to  the  following  periods : 

Prospect  Ridge :  Cambrian  and  Siluriau. 

Fish  Creek  Mountains :  Silurian. 

Silverado  and  County  Peak:  Silurian  and  Devonian. 

Mahogany  Hills:  Devonian. 

Diamond  Mountain :  Devonian  and  Carboniferous. 

Carbon  Ridge  and  Spring  Hill:  Carboniferous. 

In  the  subjoined  section,  which  may  be  best  designated  as  the  Eureka 
section,  the  relative  thickness  and  general  lithological  characters  are  given 
for  all  the  geological  divisions  which  have -been  made  of  the  sedimentary 
rocks.  A  plane  of  unconformity  in  the  Silurian  is  indicated  by  double 
dividing  lines  between  the  Eureka  quartzite  and  Lone  Mountain  limestone. 


EUEEKA  SECTION. 

Eureka  Section,  Nevada,  30,000  feet. 


13 


I 

0>' 

i 

0 

500 

Light  colored  blue  and  drab  limestones. 

2,000 

Coarse  and  fine  conglomerates,  with  angular  fragments  of  chert; 
reddish  yellow  sandstone. 

layers  of 

3,800 

Heavy  bedded  dark  blue  and  gray  limestone,  with  intercalated 
chert  ;  argillaceous  beds  near  the  base. 

bauds  ot 

3,000 

Massive  gray  and  brown  quartzite,  with  brown  and  green  shales  at  the 
summit. 

DEVONIAN,  8,000  feet. 

White  Pine  shale  

2,000 

Black  argillaceous  shales,  more  or  less  arenaceous,  with  intercalations  of 
red  and  reddish  brown  friable  sandstone,  changing  rapidly  with  the 
locality;  plant  impressions. 

6,000 

Lower  horizons  indistinctly  bedded,  saccharoidal  texture,  gray  color,  pass- 
ing up  into  strata  distinctly  bedded,  brown,  reddish  brown,  and  gray 
in  color,  frequently  finely  striped,  producing  a  variegated  appearance. 
The  upper  horizons  are  massive,  well  bedded,  bluish  black  in  color  ;  highly 
fossiliferous. 

SILURIAN.  5,000  feet. 

Lone  Mountain  limestone  1,  800 

Black,  gritty  beds  at  the  base,  passing  into  a  light  gray  siliceous  rock,  with 
all  traces  of  bedding  obliterated;  Trenton  fossils  at  the  base;  Haly  sites 
in  the  upper  portion. 

500 

Compact,  vitreous  quartzite,  white,  blue,  passing  into  reddish  tints  near 
the  base;  indistinct  bedding. 

2,700 

Inters  tratitied  limestone,  argillites,  and  arenaceous  beds  at  the  base,  pass- 
ing into  purer,  fine  grained  limestone  of  a  bluish  gray  color,  distinctly 
bedded  ;  highly  fossiuferous. 

CAMBRIAN,  7,700  feet. 

350 

Yellow  argillaceous  shale,  layers  of  chert  nodules  throughout  the  bed,  but 
more  abundant  near  the  top. 

1.200 

Dark  gray  and  granular  limestone;  surface  weathering,  rough  and  ragged  ; 
only  slight  traces  of  bedding. 

1,600 

Yellow  and  gray  argillaceous  shales,  passing  into  shaly  limestone;  near 
the  top,  iuterstratined  layers  of  shale  and  thinly  bedded  limestones. 

Prospect  Mountain  limestone.  .  . 

3,050 

Gray,  compact  limestone;  lighter  in  color  than  the  Hamburg  limestone, 
traversed  with  thin  seams  of  calcite  ;  bedding  planes  very  imperfect. 

Prospect  Mountain  quartzite  .  .  . 

1.500 

Bedded  brownish  white  quartzites,  withering  dark  brown;  ferruginous 
near  the  base;  intercalated  thin  layers  <»t  arenaceous  shales;  bvds  whiter 
near  lite  summit. 

NOTE.— I'lane  of  uucouforiuily  indicated  by  double  dividing  line. 


14  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

Longitudinal  Faults.— The  most  profound  faults,  those  which  mark  the 
greatest  amount  of  displacement  and  have  exerted  the  most  influence  in 
producing  the  present  structural  features  of  the  region,  cross  the  mountains 
at  varying  intervals  with  an  approximately  north  and  south  trend  from  Fish 
Creek  Basin  to  Diamond  Valley.  These  faults  constitute  the  principal 
factors  in  outlining  the  individual  orographic  blocks,  and  probably  from  the 
beginning  of  mountain  building  up  to  the  present  time,  and  certainly 
through  the  Tertiary  period,  have  played  a  most  important  part  in  their 
development.  The  amount  of  displacement  along  those  faults  that  extend 
the  entire  length  of  the  mountains  is  very  great,  measuring  at  some  points 
in  their  course  as  high  as  13,000  feet. 

The  four  principal  lines  of  displacement  are  the  Spring  Valley  and 
Sierra  fault,  on  the  west  side  of  Prospect  Ridge;  the  Hoosac  fault,  separat- 
ing Prospect  Ridge  from  Spring  Hill  and  Carbon  Ridge;  the  Pinto  fault, 
lying  between  the  Spring  Hill  and  Carbon  Ridge  on  the  one  side  and  the 
County  Peak  and  Silverado  Mountain  block  on  the  other,  and  the  Rescue 
fault,  on  the  east  side  of  the  latter  block.  These  main  faults  will  be  de- 
scribed here.  Numerous  other  longitudinal  faults,  while  they  express 
powerful  orographic  movements,  are  more  restricted  in  their  influence  and 
confined  within  the  limits  of  one  or  the  other  mountain  blocks  into  which 
the  country  is  broken  up.  They  will  be  mentioned  with  more  or  less  detail 
when  describing  the  particular  region  in  which  they  occur. 

Spring  Valley  and  sierra  Fault — The  Spring  Valley  fault  adheres  closely  to 
the  west  base  of  Prospect  Ridge  and  sharply  defines  the  ridge  both  in 
physical  and  geological  structure  from  the  Mahogany  Hills  on  the  opposite 
side  of  the  narrow  valley  which  has  given  its  name  to  the  fault,  and  through 
which  the  line  of  the  displacement  runs  Along  the  base  of  Prospect  Ridge 
the  oldest  Cambrian  strata  yet  recognized  in  the  Great  Basin  come  up 
against  the  fault  and  are  separated  by  it  from  the  Silurian  and  Devonian  beds 
which  form  the  mountains  to  the  west.  On  the  west  side  of  the  fault 
and  opposite  Prospect  Peak,  the  culminating  point  on  the  ridge,  the  Eureka 
quartzite  of  Spanish  Mountain  is  exposed  against  the  fault  line.  The  strati- 
graphical  position  of  the  Eureka  quartzite  along  the  Hoosac  fault  on  the 
east  base  of  Prospect  Ridge,  where  it  overlies  the  great  development  of 


HOOSAC  FAULT.  15 

Cambriam  strata  and  the  Pogonip  limestone  of  the  Silurian,  thoroughly 
well  establishes  the  fact  that  there  occurs  a  displacement  of  over  1 1,000 
feet  along  the  Spring  Valley  fault  at  the  west  base  of  Prospect  Peak.  At 
the  southwest  corner  of  Prospect  Peak  a  fault  runs  up  the  steep  slope  of 
the  mountain  with  a  somewhat  irregular  course  till  reaching  the  summit, 
where  it  joins  the  Sierra  fault  on  the  south  side  of  the  peak.  This  cross 
fault  going  up  the  side  of  the  mountain  has  been  designated  the  Prospect 
Peak  fault.  By  this  fault  the  entire  series  of  beds  belonging  to  the  Cam- 
brian quartzite  are  abruptly  cut  off,  and  Silurian  strata  are  found  lying 
unconformably  against  it.  The  Sierra  fault  resumes  the  longitudinal  trend 
and,  with  an  occasional  break  in  its  course,  continues  southward  until  the 
Cambrian  ridge  which  it  limits  on  the  west  gradually  sinks  below  the  plain. 
Along  the  Sierra  fault  the  Eureka  quartzite  for  the  greater  part  of  the  dis- 
tance lies  next  the  Prospect  Mountain  limestone,  the  Cambrian  quartzite 
not  being  exposed  south  of  Prospect  Peak;  otherwise  the  Sierra  fault 
presents  much  in  common  with  that  of  the  Spring  Valley,  having  the  same 
general  trend,  and  with  the  Cambrian  on  one  side  and  the  Silurian  on  the 
other.  From  many  points  of  view  these  three  faults,  the  Spring  Valley, 
Prospect  Peak,  and  Sierra,  may  be  regarded  as  a  single  line  of  faulting 
making  a  sharp  turn  or  fold  in  its  course  up  the  steep  slope  of  Prospect 
Peak  and  on  reaching  the  summit  of  the  ridge,  swinging  back  again  to  the 
normal  north  and  south  direction.  The  three  faults  taken  together  extend 
the  entire  length  of  the  mountains,  from  Diamond  to  Fish  Creek  valleys, 
completely  isolating  the  Cambrian  strata  from  the  Silurian  and  Devonian 
lying  to  the  westward.  As  evidence  of  the  continuity  of  the  faults,  it  may 
be  stated  that  along  the  course  of  the  Sierra  fault  on  the  summit  of  the  ridge, 
no  displacement  of  strata  has  been  recognized  north  of  its  junction  with  the 
Prospect  Peak  fault,  the  base  of  the  Cambrian  limestone  resting  conform- 
ably on  the  summit  of  the  Cambrian  quartzite. 

Hoosac  Fault.— A  sharp  contrast  between  the  Hoosac  fault  lying  on  the 
east  side  of  the  Prospect  Ridge  and  the  Spring  Valley  fault  on  the  west  side, 
is  shown  by  the  large  amount  of  lavas  that  have  broken  out  along  the 
former  and  that  are  wholly  wanting  along  the  latter.  Indeed,  the  course  of 
the  Hoosac  fault  can  be  traced  only  approximately,  owing  to  the  vast  ac- 


16  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

cumulation  of  these  lavas  poured  out  along  the  line  of  displacement,  in 
places  concealing  the  underlying  rocks  for  considerable  distances  on  both 
sides.  Within  certain  limits,  however,  there  is  no  great  difficulty  in  de- 
termining its  main  course,  as  on  the  one  side  only  Silurian  rocks  occur, 
while  on  the  other  all  the  beds  known  to  be  in  their  true  structural  position 
belong  to  the  Lower  Coal-measures.  At  the  southern  end  of  the  moun- 
tains, where  the  sedimentary  beds  emerge  from  beneath  the  Quaternary, 
the  fault  is  completely  obscured  by  rhyolite  flows  that  flank  the  slopes  of  a 
long  ridge'  of  Eureka  quartzite,  the  uppermost  member  of  the  Prospect 
Ridge  series  just  to  the  westward.  Opposite  Pinto  Peak,  where  the  rhyo- 
lite flows  are  of  exceptional  width  and  of  great  thickness,  no  indications  of 
its  trend  are  visible,  and  not  until  east  of  Hoosac  Mountain  do  the  sedi- 
mentary rocks  rise  above  the  rhyolite.  At  Hoosac  Mountain  occurs  the 
only  case  of  Silurian  beds  found  on  the  east  side  of  the  fault  line,  and  this 
is  more  apparent  than  real,  as  it  is  rather  an  instance  where  a  body  of 
quartzite  has  been  thrust  eastward  by  powerful  volcanic  forces  and  lies 
superimposed  either  upon  igneous  rocks  or  a  body  of  Carboniferous  lime- 
stone. It  is  probably  only  a  thin  capping  of  quartzite,  and  evidently  out 
of  place,  as  just  eastward  of  it  the  limestones  may  be  seen  in  their  true 
position. 

Proceeding  northward  the  Eureka  quartzite,  at  the  base  of  Hamburg 
Ridge,  marks  the  fault  on  the  west,  and  in  direct  contact  with  it  lies  the 
Lower  Coal-measures  of  Spring  Hill  Ridge,  a  contact  which  is  maintained 
nearly  to  New  York  Canyon,  only  here  and  there  slightly  obscured  by 
Quaternary  accumulations.  At  New  York  Canyon  the  fault  bifurcates,  one 
branch  turning  to  the  northeast  and  the  other  to  the  northwest,  the  easterly 
branch  being  the  main  one  and  retaining  the  name,  Hoosac  fault.  The 
fault  trending  to  the  northeast  still  continues  to  mark  the  boundary  between 
the  Silurian  and  Carboniferous,  following  the  course  of  New  York  Canyon, 
and  from  here  northward  the  contact  is  nowhere  obscured  by  outbursts  of 
lava,  the  Lone  Mountain  Silurian  of  McCoy's  Ridge  being  found  on  the 
northwest  side  of  the  displacement,  with  the  Lower  Coal-measures  on  the 
southeast.  A  short  distance  beyond  the  entrance  to  New  York  Canyon, 
near  the  Richmond  smelting  works,  the  fault  ceases  to  be  traceable  toward 


KUBY  HILL  AND  PINTO  FAULTS.  17 

the  north.  No  precise  measurement  of  the  amount  of  displacement  along 
the  east  base  of  Prospect  Ridge  can  be  given,  but  estimating  it  from  the 
known  thickness  of  the  strata  lying  between  the  summit  of  the  Eureka 
quartzite  and  the  base  of  the  Lower  Coal-measures  as  given  in  the  Eureka 
section,  we  have  a  vertical  movement  of  12,800  feet.  Now,  if  we  suppose, 
and  it  seems  highly  probable,  that  there  are  300  or  400  feet  of  limestones 
beneath  the  beds  exposed  at  the  surface,  and  that  the  upper  portion  of 
the  Eureka  quartzite  is  also  wanting,  we  have  a  displacement  of  over  13,000 
feet.  Probably  the  vertical  movement  at  its  maximum  displacement 
amounted  to  more  than  2J  miles,  lying  wholly  within  Paleozoic  rocks. 

Ruby  Hill  Fault.— The  branch  fault  which  leaves  the  main  one  just  after 
it  enters  New  York  Canyon  from  the  south  trends  northwesterly  across  the 
slope  of  Prospect  Ridge,  thence  across  Ruby  Hill,  probably  connecting 
with  the  Spring  Valley  fault  although  it  has  never  been  traced  beyond  the 
Richmond  and  Albion  mines.  It  has  been  designated  the  Ruby  Hill  fault. 
On  the  atlas  sheet  its  course  is  indicated  only  a  short  distance  beyond  the 
Jackson  fault,  its  true  position  on  Ruby  Hill  not  having  been  accurately 
located  until  after  the  printing  of  the  map.  Although  the  Ruby  Hill  fault 
possesses  features  of  great  economic  importance  bearing  upon  the  ore  de- 
posits of  the  district,  it  is  by  no  means  so  profound  a  displacement  as  the 
Hoosac  and  is  measured  by  hundreds  instead  of  thousands  of  feet.  The 
dynamic  movements  which  produced  it  have  not  influenced  in  any  marked 
manner  the  structural  features  of  the  country,  presenting,  in  this  respect, 
the  greatest  possible  contrast  with  the  main  Hoosac  fault.  There  is  some 
reason  for  the  opinion  that  the  Ruby  Hill  fault  is  of  later  date  than  the  main 
fault,  and  belongs  to  the  period  of  Tertiary  eruptions.  A  more  detailed 
description  of  this  fault  will  be  found  in  the  chapter  devoted  to  the  discus- 
sions of  the  ore  deposits. 

Pinto  Fault.— This  fault  is  situated  about  2  miles  to  the  east  and  nearly 
parallel  with  the  Hoosac  fault,  which  it  closely  resembles  in  structural 
features.  Like  the  Hoosac,  its  course  can  not  be  traced  with  precision,  yet 
the  geological  characters  are  so  distinctive  that  there  exist  scarcely  any 
difficulties  in  the  way  of  determining  its  main  trend  across  the  mountains 
as  it  sharply  defines  the  boundary  between  the  elevated  County  Peak  and 
MON  xx 2 


18  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

Silverado  block  on  the  one  side  and  the  depressed  Spring  Hill  and  Carbon 
Ridge  block  on  the  other.  On  the  west  side,  wherever  the  volcanic  and 
detrital  material  fails  to  conceal  the  underlying  rocks  only  Carboniferous 
strata  are  exposed,  whereas,  on  the  opposite  side  Silurian  strata  every- 
where rise  above  the  fault  line  in  bold  and  abrupt  ridges. 

Starting  from  the  southern  end  of  the  mountains  the  fault  follows  up 
Pinto  Valley,  with  Carbon  Ridge  on  the  west  and  English  Mountain  on  the 
east,  the  intermediate  valley  being  filled  with  pumices  and  tuffs.  Not 
until  nearly  opposite  Dome  Mountain  do  the  sedimentary  beds  on  both 
sides  of  the  fault  come  in  direct  contact  at  the  surface,  but  here  we  find 
the  Lower  Coal-measures  limestone  brought  up  unconformably  against  the 
Lone  Mountain  limestone.  From  here  a  deep,  narrow  limestone  gorge 
extends  northward,  along  which  the  limestones  of  the  two  different 
epochs  stand  out  boldly  on  opposite  walls,  the  direction  of  the  gorge  coin- 
ciding with  the  line  of  the  fault.  Where  the  drainage  channel  following 
the  gorge  turns  abruptly  toward  the  west  the  Eureka  quartzite  comes  in 
beneath  the  Lone  Mountain  strata,  but  the  fault,  without  deviating  in  the 
least  from  its  course,  continues  northward  with  the  Carboniferous  limestone 
still  on  the  west  side.  A  short  distance  farther  northward  the  sedimentary 
strata  are  buried  beneath  the  lavas  of  Richmond  Mountain.  The  vertical  dis- 
placement along  the  Pinto  is  probably  quite  as  great  as  that  found  along 
the  Hoosac  fault;  the  same  geological  horizons  are  here  brought  into  juxta- 
position, although  higher  beds  form  the  contact  along  the  Pinto  fault,  and 
at  Carbon  Ridge  the  Weber  conglomerates  come  in  as  the  uppermost  beds. 
The  enormous  development  of  Devonian  strata  and  the  Diamond  Peak 
quartzite,  which,  as  shown  by  the  section,  have  an  estimated  thickness  of 
11,000  feet,  is  wholly  wanting. 

Rescue  Fault.— About  2|  miles  east  of  the  Pinto  fault,  and  on  the  east  side 
of  the  Silverado  and  County  Peak  block,  runs  the  equally  persistent  but 
less  profound  Rescue  fault.  It  derives  its  name  from  Rescue  Canyon, 
which,  in  turn,  owes  its  origin  primarily  to  the  fault.  The  canyon,  a  longi- 
tudinal mountain  valley  nearly  2  miles  in  length,  opening  out  into  Fish 
Creek  Basin,  is  now  occupied  for  the  entire  distance  by  rhyolite  extrava- 
sated  along  the  course  of  the  fault,  At  the  head  of  the  canyon  the  rhyolite 


PROSPECT  RIDGE.  19 

gives  out  and  the  fault  enters  the  Nevada  limestone  with  a  course  a  little 
east  of  north,  and  follows  along  under  the  abrupt  east  wall  of  Sugar  Loaf. 
A  short  distance  beyond  Sugar  Loaf  the  fault  coincides  with  the  contact  of 
the  Nevada  limestone  with  the  White  Pine  shale,  maintaining  this  course 
until  both  the  limestone  and  shale  pass  beneath  the  basalt  tableland  toward 
the  north.  That  the  fault  continues  beyond  this  point  beneath  the  basalt 
is  clearly  established  by  geological  structure,  the  Devonian  strata  of  County 
Peak  passing  under  the  tableland  on  the  west  side  and  the  Weber  con- 
glomerate and  Upper  Coal-measures  dipping  toward  it  and  passing  beneath 
it  on  the  east.  There  can  be  no  doubt  that  the  Rescue  fault  sharply  defines 
a  great  physical  break  separating  the  County  Peak  from  the  Diamond  Peak 
block.  After  entering  the  region  occupied  by  the  basalt  field,  there  is  110 
means  of  determining  the  precise  course  of  the  fault,  everything  being 
obscured  by  recent  lavas.  Upon  leaving  the  basalt  area  the  fault  probably 
follows  along  the  east  base  of  Richmond  Mountain,  but  is  hidden  beneath 
the  andesitic  rocks  that,  flowing  eastward,  rested  against  the  base  of  the 
gently  inclined  slopes  of  the  Upper  Coal-measure  limestones  of  the  Dia- 
mond Range.  Beneath  the  lavas  the  trend  of  the  fault,  while  in  a  great 
degree  conjectural,  can  not  vary  far  from  the  course  of  the  contact  between 
the  Nevada  limestone  and  the  White  Pine  shale  as  exposed  to  the  south  and 
the  line  of  the  Carboniferous  rocks  to  the  north  and  east.  In  the  region  of 
the  volcanic  rocks  the  displacement  along  the  fault  can  not  be  measured, 
although  it  must  be  very  great,  as  is  shown  by  the  Devonian  beds  on  the 
one  side  and  the  upper  members  of  the  great  development  of  the  Carbon- 
iferous sediments  on  the  other.  South  of  the  basalt  the  fault  runs  wholly 
within  the  limits  of  the  upper  portion  of  the  Nevada  limestone,  or  else  at 
the  base  of  the  White  Pine  shales.  Nowhere  along  its  entire  course,  from 
Packer  Basin  to  Fish  Creek  Valley,  does  the  downthrow  apparently  exceed 
3,000  feet  of  vertical  displacement. 

GEOGRAPHIC    BLOCKS. 

Prospect  Ridge.-This  ridge  stands  out  as  the  most  prominent  orographic 
feature  of  the  Eureka  Mountains.  It  is  situated  in  the  very  center  of  the 
mountains  and  presents  a  bold,  serrated  outline,  extending  with  an  approx- 


20  GEOLOGY  OF  THE   KUKKKA   DISTRICT. 

imately  north  and  south  trend  from  Diamond  Valley  to  the  Fish  Creek 
Basin.  From  Diamond  Valley  the  northern  slopes  rise  gradually  out  of  the 
plain  to  the  summit  of  Ruby  Hill,  beyond  which  the  mountains  assume  a 
more  rugged  aspect,  continuing  southward  in  an  unbroken  ridge  until  cut 
off  sharply  by  eruptive  masses  or  concealed  beneath  Quaternary  accumula- 
tions of  the  valley. 

As  already  described,  this  orographic  block  is  sharply  outlined  along  it:? 
entire  eastern  base  by  the  Hoosac  fault,  evidence  of  which  is  shown  in  the 
geological  character  of  the  opposite  walls  and  in  the  extravasated  rocks  that 
have  broken  out  along  the  line  of  dislocation.  The  Spring  Valley,  Prospect 
Mountain,  and  Sierra  faults  as  clearly  define  it  on  the  west,  except  that 
along  the  entire  length  of  these  combined  faults  no  lavas  reach  the  surface. 
The  Sierra  fault  marks  a  more  decided  geological  than  topographical  break, 
since  along  the  displacement  an  intricate  and  confused  mass  of  mountains 
unites  Prospect  Ridge  with  the  country  to  the  west  of  it,  the  Silurian  and 
Devonian  rocks  resting  against  the  Prospect  Mountain  limestone  high  up  on 
the  summit  without  any  intervening  valley  or  depression.  With  these 
clearly  defined  boundaries  the  Prospect  Ridge  block  measures  10  miles  in 
length  and  across  its  broadest  development,  in  the  region  of  Prospect  Peak, 
between  2  and  2^  miles  in  width.  Topographically  this  mountain  block  is 
quite  simple — a  longitudinal  ridge  rising  abruptly  on  the  west  side  with 
Prospect  Peak,  the  culminating  point,  descending  for  2,500  feet  toward 
Spring  Valley  with  an  average  slope  of  30°,  but  on  the  east  side  falling 
away  much  more  gradually  and  with  far  less  regularity  towards  the  Hoosac 
fault. 

In  structure  Prospect  Ridge  is  an  anticlinal  fold,  and  affords  an  admir- 
able example  of  such  structure,  accompanied  by  profound  north  and  south 
faults  approximately  parallel  with  the  strike  of  the  beds.  The  axis  of  the 
fold  lies  wholly  on  the  western  side  of  the  ridge  and  is  well  shown  on  the 
slopes  of  Prospect  Peak,  the  beds  on  both  sides  of  the  axial  plane  standing 
inclined  at  an  angle  of  nearly  80°.  While  the  crest  of  the  ridge  trends 
north  and  south,  the  axis  of  the  fold,  striking  west  of  north,  follows  obliquely 
down  the  slope  and  is  finally  lost  in  the  valley  toward  the  west.  The  rocks 
which  constitute  this  great  body  of  folded  strata  between  the  two  lines  of 


FISH  CREEK  MOUNTAINS.  21 

faulting  present  a  conformable  series  of  sediments  inclined  throughout  their 
entire  thickness  at  angles  seldom  less  than  75°. 

From  the  axis  of  the  anticline,  near  the  summit,  on  the  west  side  of 
Prospect  Peak,  to  the  Hoosac  fault  along  the  eastern  base  of  the  ridge, 
there  is  exposed  a  series  of  strata  measuring  nearly  10,000  feet  in  thickness, 
and  wholly  made  up  of  Cambrian  and  Silurian  rocks.  The  axis  of  this 
fold  occurs  in  the  Prospect  Mountain  quartzite,  the  underlying  member  of 
the  Cambrian,  and  is  in  turn  overlain  successively  by  the  Prospect  Moun- 
tain limestone,  Secret  Canyon  shale,  Hamburg  limestone,  Hamburg  shale, 
Pogonip  limestone,  and  Eureka  quartzite.  Along  the  Hoosac  fault  the 
Eureka  quartzite  is  well  exposed  at  Caribou  Hill,  McCoy's  Ridge,  Hoosac 
Mountain,  and  the  narrow  ridge  east  of  Round  Top. 

Prospect  Ridge  affords  the  grandest  section  of  Cambrian  rocks  yet 
recognized  in  the  Great  Basin,  and  with  the  exception  of  one  or  two  insig- 
nificant exposures  of  slight  importance  east  of  the  Sierra  fault,  the  rocks  of 
this  period  are  confined  to  this  orographic  block.  Section  CD-EF  (atlas 
sheet  xni),  constructed  across  the  central  portion  of  the  Eureka  Mountains, 
intersects  Prospect  Ridge  about  3,000  feet  to  the  north  of  the  peak  at  a 
point  well  chosen  to  bring  out  the  anticlinal  structure  of  the  uplifted  block 
and  its  relations  to  the  fault  lines.  There  is  represented  on  PI.  n,  Fig.  4,  a 
geological  section  drawn  at  right  angles  to  the  strike  of  the  beds  across  the 
culminating  point  of  Prospect  Peak,  from  Spring  Valley  to  the  Hoosac 
fault.  The  Prospect  Mountain  limestone  is  here  shown  capping  the  peak 
and  the  entire  east  slope,  and  it  is  again  exposed  at  the  base  of  the  ridge  on 
the  west  side  of  the  anticline,  rising  above  the  detrital  material  of  Spring 
Valley.  In  Fig.  3  of  the  same  plate  will  be  found  a  section  of  the  same 
strata  across  Ruby  and  Adams  Hills.  Here  the  beds  are  inclined  at  a  much 
lower  angle,  otherwise  the  structural  features  and  succession  of  strata  are 
nearly  identical,  Ruby  Hill  coi-responding  to  Prospect  Peak  and  Adams 
Hill  to  the  Hamburg  Ridge,  with  the  intermediate  Secret  Canyon  shale 
occupying  a  depression  between  them. 

Fish  creek  Mountains.— To  the  southwest  of  the  Sierra  fault  the  character 
of  the  country  changes,  and  a  confused  and  intricate  series  of  ridges  come 
in,  presenting  a  strong  contrast  to  the  adjacent  region.  In  place  of  the 


22  GEOLOGY  OF  TIIE  EUKEKA  UISTEIGT. 

single  ridge  structure,  as  seen  toward  the  north,  the  configuration  of 
the  country  shows  a  broad,  rough  mass  of  mountains,  from  4  to  5  miles 
in  width,  of  very  diversified  topographic  forms  and  deeply  scored  by 
narrow  gorges.  In  the  region  of  Atrypa  Peak,  Gray's  Peak,  and  Lookout 
Mountain  a  classification  of  the  mountain  masses  becomes  a  matter  of  much 
difficulty,  the  orographic  structure  being  complex,  and  the  resultant  of 
forces  in  some  respects  different  from  those  which  elevated  Prospect  Ridge 
or  the  Fish  Creek  Mountains.  Southward  from  Castle  Peak  the  latter 
mountains  become  a  distinct  range,  and  with  a  north  and  south  trend  stretch 
off  southward  several  miles  beyond  the  limits  of  this  survey.  They  are 
situated  in  the  extreme  southwest  corner  of  the  Eureka  District,  and  are 
sharply  defined  by  the  broad  valley  of  Fish  Creek  on  the  one  side  and  An- 
telope Valley  on  the  other,  which  partially  disconnects  them  from  the 
Eureka  Mountains.  They  measure  about  5  miles  in  width  and  rise  over 
2,000  feet  above  the  adjoining  Quaternary  plain.  They  present  the  im- 
pressive appearance  of  a  solid  mountain  mass  gently  inclined  to  the  west, 
but  falling  off  somewhat  abruptly  on  the  east,  accompanied  by  a  steep  es- 
carpment just  beneath  and  parallel  with  the  summit  of  the  ridge.  The 
structure  is  that  of  an  anticlinal  fold  whose  axial  plane  coincides  with  the 
escarpment  along  which  there  has  been  a  downthrow  of  600  feet.  The 
origin  of  the  escarpment  is  due  to  the  faulting.  At  the  base  of  the  cliff  the 

• 

faulted  strata  are  uniformly  inclined  toward  the  valley  at  an  angle  of 
about  15°.  Along  the  west  side  of  the  anticlinal  axis  the  beds  lie  at  much 
lower  angles,  exhibiting  first  a  slight  synclinal  fold  followed  by  an  equally 
gentle  anticlinal,  beyond  which  for  nearly  2  miles  they  fall  away  with  a 
nearly  uniform  dip  toward  Antelope  Valley. 

The  Fish  Creek  Mountains  may  be  considered  as  essentially  made  up 
of  Silurian  rocks,  in  marked  contrast  with  Prospect  Ridge,  which  is,  as  has 
been  already  shown,  formed  of  Cambrian  strata  with  outlying  slopes  of 
Pogonip  limestone  and  Eureka  quartzite.  Here  are  exposed  the  two  lower 
members  of  the  Silurian  in  a  manner  which  can  hardly  be  excelled  for  sim- 
plicity of  structure  elsewhere  in  the  Great  Basin.  Nearly  all  the  more  ele- 
vated portions  of  the  mountains  consist  of  Upper  Pogonip  limestone,  the 
axis  of  the  fold  occurring  not  far  below  the  top  of  the  horizon.  The  Eureka 


MAHOGANY  HILLS.  23 

quartzite  overlies  the  limestone  on  both  sides  of  the  mountains,  but  as  the 
dip  of  the  strata  coincides  closely  with  the  inclination  of  the  western  slope, 
it  comes  to  the  surface  only  near  the  base  of  the  ridge.  As  the  strata  dip 
away  both  to  the  north  and  south  from  the  central  body  of  Pogonip  lime- 
stone, a  belt  of  the  quartzite  may  be  observed  encircling  it  on  all  sides. 
Nowhere  do  the  Fish  Creek  Mountains  expose  a  section  of  the  Pogonip 
limestone  for  more  than  one-quarter  of  its  thickness,  as  given  in  the  general 
section,  although  numerous  excellent  partial  sections  are  shown  of  the 
Upper  Pogonip  beds.  Northward  of  Bellevue  Peak,  and  in  the  region 
of  Castle  Mountain,  the  Lone  Mountain,  limestone  overlying  the  Eureka 
quartzite  comes  to  the  surface,  and  again  at  the  southern  end  of  the  range, 
but  beyond  the  limits  of  the  map. 

From  this  description,  and  by  the  aid  of  the  map  (atlas  Sheet  xi),  a  clear 
idea  may  be  obtained  of  the  broader  features  of  the  Fish  Creek  Mountains, 
and  in  the  chapters  devoted  to  the  Silurian  rocks  and  the  descriptive  geology 
there  will  be  found  the  evidences  in  detail  for  the  conclusions  presented 
here  as  to  their  age  and  structure." 

Mahogany  Hills.— The  Mahogany  Hills  are  situated  on  the  west  side  of  the 
Eureka  Mountains.  They  occupy  by  far  the  largest  area  of  any  of  the 
mountain  blocks  into  which  the  country  has  been  divided,  and  are  as  sharply 
denned  as  any  of  the  others  by  natural  physical  outlines.  Spring  Valley  and 
Canyon  serve  as  an  excellent  boundary  between  them  and  Prospect  Ridge, 
but  everywhere  else,  except  along  the  narrow  belt  which  connects  them  with 
the  Fish  Creek  Mountains,  the  broad  Quaternary  plain  rests  against  the 
upturned  edges  of  the  outlying  ridges.  From  Spring  Valley  the  Mahogany 
Hills  extend  westward,  a  mountain  mass  over  8  miles  in  width  ;  in  a  north 
and  south  direction  they  present  an  unbroken  body  of  limestone,  12  miles 
in  length.  This  broad  mountain  mass  maybe  divided  into  two  nearly  equal 
parts,  separated  by  the  level  plain  of  Dry  Lake  and  the  narrow  gorge  of 
Yahoo  Canyon,  the  lake  at  one  time  draining  northward  through  the  canyon 
into  Hayes  Valley.  The  country  to  the  east  of  the  lake  and  canyon, 
while  it  has  much  in  common  with  the  western  side,  is,  in  structural 
features,  closely  related  to  the  Pinon  Range.  This  latter  range,  which  is 
made  up  of  a  number  of  longitudinal  ridges  extending  from  the  Humboldt 


24  GEOLOGY  OP  THE  EUKEKA  DISTRICT. 

River  to  the  Eureka  Mountains,  may  be  said  to  terminate  at  the  deeply 
eroded  pass  known  as  The  Gate,  as  it  there  loses  its  distinctive  features. 
The  monoclinal  character  of  the  uplifted  ridges  is,  however,  still  maintained 
nearly  to  Spanish  Mountain,  or  until  cut  off"  by  the  Spring  Valley  fault. 

From  Dry  Lake  westward  the  mountains  rise  abruptly,  frequently  in 
steep  cliffs,  presenting  a  somewhat  monotonous  aspect  of  dark  bluish  gray 
limestone  covered  with  a  scanty  growth  of  mountain  mahogany  (Cercocarpns 
laxlifolius),  from  which  the  region  derives  its  name.  A  few  culminating 
points  attain  elevations  above  the  general  level,  but  these  gradually  fall 
away  to  the  westward  in  long  uniform  ridges,  sharply  denned  by  drainage 
channels  that  cut  down  hundreds  of  feet  into  the  limestones  with  nearly 
vertical  escarpments 

Mahogany  Hills  are  made  up  for  the  most  part  of  Nevada  limestone, 
which  everywhere  forms  all  the  more  elevated  portions.  Silurian  rocks 
occur  in  one  or  two  localities,  but  principally  at  Spanish  Mountain,  where 
the  Eureka  quartzite  is  admirably  shown,  with  all  its  peculiarities  of  struc- 
ture, overlain  by  the  Lone  Mountain  limestone,  which  in  turn  passes  con- 
formably into  the  Nevada  limestone.  For  purposes  of  stratigraphical  geol- 
ogy, the  position  of  Spanish  Mountain  is  most  fortunate,  as  its  relation  to 
the  overlying  Devonian  limestone  is  well  brought  out,  while  its  relation  to 
the  underlying  limestones  and  shales  of  the  Lower  Silurian  and  Cambrian 
is  demonstrated  beyond  question  in  both  the  Fish  Creek  Mountains  and 
Prospect  Ridge.  Spanish  Mountain  happens  to  be  the  only  area  of  Eureka 
quartzite  in  the  Mahogany  Hills.  On  the  southern  slope  of  Comb's  Peak 
the  upturned  beds  afford  an  excellent  exposure  of  the  limestones  overlying 
the  Eureka  quartzite,  and  give  a  section  of  Lone  Mountain  rocks  lower  than 
found  elsewhere,  including  a  series  of  beds  whose  geological  position  is 
determined  by  a  characteristic  Trenton  fauna.  The  relationship  of  this  fauna 
just  above  the  Eureka  quartzite  to  the  fauna  found  elsewhere  immediately 
below  the  quartzite  offers  an  important  link  in  the  paleontological  history 
of  the  Eureka  District.  One  of  the  best  sections  across  the  Nevada  limestone 
may  be  found  on  the  ridge  north  of  Modoc  Peak,  where  the  beds  throughout 
a  great  vertical  thickness  present  a  nearly  uniform  strike  and  dip,  with  but 
little  disturbance  or  dislocation.  The  Modoc  section  measures  about  5,400 feet 


SILVEEADO  AND  COUNTY  PEAK  GROUP.  25 

in  thickness.  It  is  given  in  detail  in  the  chapter  devoted  to  a  discussion  of 
the  Devonian  rocks,  on  page  66. 

Silverado  and  County  Peak  Group.— This  mountain  block  stands  almost  com- 
pletely isolated  from  the  others,  being  cut  off  by  profound  faults  on  all 
sides,  along  which  igneous  rocks  have  reached  the  surface  in  enormous 
masses.  In  this  way  it  is  clearly  outlined  from  the  Diamond  Range  on  the 
northeast  by  the  broad  basalt  table  of  Basalt  Peak  and  the  Strahlenberg, 
on  the  north  by  Richmond  Mountain,  and  on  the  west  in  great  part  from 
Carbon  Ridge  and  Spring  Hill  group  by  the  extravasated  rocks  along  the 
Pinto  fault.  A  glance  at  the  map  will  show  how  closely  these  lavas  sur- 
round the  mountains  and  there  is  good  reason  to  believe  that  if  the  Quater- 
nary deposits  along  the  foothills  were  removed  this  encircling  belt  of  lavas 
would  be  still  more  noticeable.  Here  and  there  a  few  isolated  patches 
of  lava  rise  above  the  level  of  the  plain  in  Fish  Creek  and  Newark  valleys, 
but  in  most  instances  the  exposures  occupy  too  limited  areas  to  permit 
of  their  being  located  upon  the  map.  The  outlines  of  the  knobs  and  knolls 
of  rock  partially  concealed  by  recent  deposits  indicate  their  probably  vol- 
canic origin. 

The  mountains  are  roughly  broken  up  into  three  groups — northern, 
southern  and  southeastern.  Wood  Valley,  a  relatively  broad  drainage 
channel  open  to  the  west,  and  Charcoal  Canyon,  a  narrow  but  deep  ravine 
south  of  Sentinel  Peak  on  the  east,  separate  the  two  former,  while  the  latter 
is  somewhat  isolated  by  the  deep  valley  of  Rescue  Canyon  and  an  arm  of 
Newark  Valley.  For  convenience  the  northern  region  may  be  designated 
as  the  County  Peak  Mountains,  the  southern  as  the  Silverado  group,  and 
the  region  to  the  southeast  as  the  Alhambra  Hills.  Taken  together  they 
stretch  from  Fish  Creek  Valley  to  Richmond  Mountain  and  in  an  east  and 
west  direction  from  the  Pinto  fault  to  the  Quaternary  plain. 

Between  the  two  great  lines  of  displacement,  the  Pinto  and  Rescue 
faults,  the  broad  mass  of  limestone  presents  a  gentle  synclinal  structure, 
the  beds  dipping  toward  the  center  from  both  fault  lines  and  away  from 
the  lines  of  igneous  outbursts.  The  mountains  are  almost  wholly  made  up 
of  limestones  belonging  to  the  Silurian  and  Devonian  periods,  all  the  more 
elevated  portions  being  formed  of  characteristic  strata  of  the  middle  and 


26  GEOLOGY  OF  THE  EUEEKA  DISTEICT. 

upper  portions  of  the  Nevada  limestone.  At  the  extreme  northeast  corner 
the  Eureka  quartzites  occupy  a  small  area,  but  are  of  no  special  importance 
themselves  except  in  determining  the  basal  rocks  of  this  elevated  mass  and 
the  position  of  the  overlying'  strata.  Numerous  narrow  gorges  with  mural- 
like  faces  cut  deeply  into  the  limestones,  affording  excellent  comparative 
sections  across  the  strata,  datum  points  being  readily  established  by  the 
brown,  red  and  gray  beds  of  the  middle  Devonian.  Represented  in  this 
uplifted  mass  occur  between  6,000  and  8,000  feet  of  limestones.  That  the 
upper  beds  of  the  Nevada  epoch  are  represented  here  is  shown  just  to  the 
east  of  Sugar  Loaf  and  Island  Mountain  where  the  White  Pine  shales  lie 
conformably  upon  the  uppermost  beds  of  limestone. 

Diamond  Mountains.— This  range  is  one  of  the  best  denned  mountain  up- 
lifts on  the  Nevada  plateau,  extending  40  miles  along  the  east  side  of  Dia- 
mond Valley.  Only  the  southern  end  of  it,  however,  in  the  northeast  corner 
of  the  map,  comes  within  the  limits  of  this  survey,  as  the  range  properly 
terminates  with  Newark  Mountain.  Its  immediate  proximity  to  the  County 
Peak  limestones,  from  which  it  is  separated  only  by  an  overflow  of  igneous 
rocks,  relates  it  in  the  closest  possible  manner  with  the  Eureka  Mountains. 
Diamond  Peak  (10,637  feet),  the  highest  and  broadest  in  the  range,  lies 
within  the  limit  of  this  survey,  and  the  geological  structure  and  continuity 
of  beds  exposed  upon  the  flanks  of  both  Diamond  Peak  and  Newark  Moun- 
tain, add  greath'  to  our  knowledge  of  the  sequence  of  Paleozoic  sediments. 
For  the  greater  part  of  its  length  Carboniferous  rocks  flank  both  sides  of  the 
Diamond  Range,  and,  as  is  so  often  the  case  tliroughout  Nevada,  no  beds 
immediately  underlying  them  had  previously  been  recognized  toward  the 
north.  Here,  however,  Newark  Mountain  consists  exclusively  of  Devonian 
rocks  passing  beneath  the  east  base  of  Diamond  Peak,  where  they  are  con 
formably  overlain  by  an  immense  thickness  of  Carboniferous  beds.  New- 
ark Mountain  rises  abruptly  out  of  the  plain  and  offers  a  typical  example, 
so  common  in  the  Great  Basin,  of  an  anticlinal  ridge  with  one  side  of  the 
fold  dropped  down  along  the  line  of  the  axial  plane.  In  this  instance  the 
downthrow  lies  on  the  east  side  and  the  mountain  presents  along  the  summit 
a  bold  escarpment  1,000  feet  in  height,  facing  Newark  Valley.  At  the  base 
of  the  escarpment  easterly  dipping  beds  come  in,  and  dark  blue  massive  lime- 


DIAMOND  MOUNTAINS.  27 

stones  of  the  Upper  Devonian  form  the  remainder  of  the  steep  slope  for 
about  1,000  feet  and  then  stretch  far  out  into  the  valley  in  a  line  of  low 
hills  and  isolated  Imttes,  still  dipping  toward  the  east.  The  entire  western 
side  of  the  mountain,  including  the  summit  of  the  ridge,  dips  uniformly 
toward  the  west,  and  is  in  turn  overlain  by  the  White  Pine  shales  through 
which  Hayes  Canyon  has  been  eroded.  On  the  north  side  of  Newark 
Mountain  these  flexible  shales  curve  around  to  the  northeast  and  form  the 
east  base  of  Diamond  Peak,  only  the  uppermost  beds  of  the  Nevada  lime- 
stone here  appearing  above  the  level  of  the  valley,  the  remaining  portion 
of  the  Devonian  beds  upon  both  sides  of  the  fold  having  dropped  completely 
out  of  sight. 

Diamond  Peak  rises  above  Newark  Valley  over  4,000  feet,  with  an 
exceptionally  steep  slope,  the  White  Pine  shales  presenting  smooth  rounded 
ridges  along  the  base  of  the  mountain.  The  shales  are  overlain  by  a  great 
thickness  of  rough  and  rugged  Diamond  Peak  quartzites,  followed  by  the 
Lower  Coal-measure  limestones  which  for  a  long  distance  form  the  summit 
of  the  ridge.  In  its  structure  the  Diamond  Range  is  in  strong  contrast  with 
the  anticlinal  structure  of  Newark  Mountain,  presenting  a  synclinal  fold 
whose  axis  lies  in  the  Lower  Coal-measures.  The  identical  series  of  beds 
found  dipping  into  the  peak  on  the  east  side  come  in  again  on  the  west 
side,  but  with  a  reverse  dip,  except  that  the  White  Pine  shales  are  not 
brought  to  the  surface,  owing  to  a  longitudinal  fault  which  extends  along 
the  west  side  of  Diamond  Peak,  completely  cutting  them  off  and  bringing 
up  still  higher  Carboniferous  formations  than  those  found  near  the  summit. 
From  the  axis  of  the  anticline  on  the  east  slope  of  Newark  Mountain  diag- 
onally across  Diamond  Peak  there  is  exposed  an  admirable  section,  includ- 
ing Nevada  limestones,  White  Pine  shales,  Diamond  Peak  quartzites,  and 
Lower  Coal-measure  limestones.  The  geological  importance  of  this  section 
lies  in  the  fact  that  it  offers,  across  the  middle  of  the  Paleozoic  rocks,  a  con- 
formable and  continuous  series  of  beds  rarely  found  elsewhere,  uniting  the 
upper  Paleozoic  with  the  great  development  of  Silurian  and  Cambrian  rocks 
beneath.  From  Bold  Bluff,  at  the  southern  end  of  Diamond  Peak,  the  New- 
si  rk  fault  brings  the  Lower  Coal-measures  against  the  White  Pine  shales,  the 
entire  development  of  Diamond  Peak  quartzite  having  been  displaced  along 


28  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

the  west  side  of  Newark  Mountain.  North  of  Newark  Mountain,  however, 
the  limestones  occupy  their  true  geological  position,  overlying  the  quartzite 
and  dipping  westerly. 

Alpha  Ridge  for  its  entire  length  is  made  up  of  Lower  Coal-measure 
limestones  uniformly  inclined  toward  the  west  and  in  turn  overlain  by  the 
Weber  conglomerates  and  Upper  Coal-measures.  In  the  Weber  conglom- 
erates there  is  a  synclinal  and  anticlinal  fold,  the  latter  being  well  shown  in 
long  narrow  ridges  stretching  in  north  and  south  lines  parallel  with  the 
bedding.  Of  the  Upper  Coal-measures  there  occurs  only  a  limited  expo- 
sure above  the  conglomerates,  but  they  are  admirably  displayed  with  their 
stratigraphical  position  well  brought  out  and  their  geological  age  deter- 
mined from  ample  paleontological  evidence. 

In  the  area  north  of  Newark  Canyon,  stretching  northward  as  far  as 
the  limit  of  the  map  and  west  of  the  Alpha  fault,  a  north  and  south  fault 
011  the  west  side  of  Alpha  Peak  ridge,  occurs  an  inclined  table  "wholly 
made  up  of  Upper  Coal-measure  limestones.  Its  identity  upon  both  lith- 
ological  and  paleontological  grounds,  with  the  body  of  Carboniferous  lime- 
stones overlying  the  Weber  conglomerates  south  of  Newark  Canyon  seems 
conclusive,  and  the  finding  of  Carboniferous  species  unlike  those  known  to 
occur  in  the  Lower  Coal-measures  at  Eureka  and  characteristic  of  the  Upper 
Coal-Tneasures  elsewhere  establishes  the  geological  position  of  these  beds. 

Carbon  Ridge  and  Spring  Hill  Group.— This  block  OCCUpieS   a   far   less    COUSpicU- 

ous  position  than  any  of  the  others,  and  seen  from  any  commanding 
point  of  view  it  would  not  be  in  the  least  likely  to  attract  attention  as  a 

I 

prominent  physical  feature  of  the  country.  Unlike  the  adjoining  uplifted 
blocks  which  rise  boldly  out  of  the  plain,  this  one  has  rather  the  appear- 
ance of  a  depressed  region  without  any  persistent  or  distinctive  character- 
istics. Nevertheless,  it  is  sharply  defined,  geologically,  by  parallel  lines  of 
displacement,  the  Hoosac  and  Pinto  faults.  On  the  one  side  rises  Prospect 
Ridge  and  on  the  other  rises  the  broad  mass  of  County  Peak  and  Silverado 
Mountains.  This  relatively  depressed  block  measures  6J  miles  in  length, 
but  between  the  faults  has  an  average  width  of  only  If  miles.  Estimating 
from  the  thicknesses  of  the  different  epochs  given  in  the  Eureka  section 
both  faults  show  profound  vertical  displacements  of  12,000  to  15,000  feet. 


rARBON  RIDGE  AND  SPUING   HILL  GROUP.  29 

Embraced  within  these  lines  of  faulting  only  Carboniferous  beds  are  exposed, 
whereas  the  inclosing  outer  walls  on  botli  sides  consist  of  Silurian  rocks 
traceable  the  entire  length  of  the  mountains  except  where  concealed  by 
volcanic  overflows.  Fissures  along  these  fault  lines  have  served  as  conduits 
for  extravasated  lavas,  through  which  have  poured  out,  either  upon  one  side 
or  the  other,  vast  accumulations  of  volcanic  material,  for  nearly  the  entire 
length  of  the  mountains.  So  extensive  have  been  these  flows  over  the 
Carboniferous  rocks  that  not  oidy  have  the  fault  planes  become  obscured, 
but  large  areas  of  the  sedimentary  beds  lie  concealed  beneath  the  lavas, 
while  in  the  region  of  the  Hoosac  Mountain  they  have  so  spread  out 
over  the  country  as  to  completely  bury  all  the  underlying  rocks  between 
the  two  faults.  Naturally  such  an  amount  of  volcanic  energy  displayed 
all  along  the  line  has  broken  and  dislocated  the  strata,  caused  minor  fault- 
ings  and  displacements,  and  over  much  of  the  area  rendered  it  difficult,  if 
not  impossible,  to  work  out  the  structural  relations  of  the  exposed  beds. 
Many  fractures  and  breaks  in  the  inclosed  rocks,  although  not  of  any  great 
magnitude,  are  frequently  sufficient  to  render  any  precise  measurement  of 
the  beds  impossible,  the  amount  of  faulting  being  undeterminable.  On 
the  other  hand  great  blocks  of  strata  have  been  tilted  up  at  high  angles 
with  only  slight  disturbances,  affording  fairly  good  cross-sections. 

The  volcanic  rocks  separate  the  sedimentary  beds,  which  otherwise 
would  form  a  continuous  body,  into  two  or  more  distinct  areas,  the  northern 
known  as  the  Spring  Hill  group  and  the  southern  as  the  Carbon  Ridge, 
while  between  them  lies  a  much  smaller  area  of  limestones  every  where  sur- 
rounded bv  eruptive  rocks.  The  middle  area  serves  in  a  measure  to  connect 
the  other  two,  the  same  beds  found  here  occurring  both  north  and  south. 
Across  the  southern  end  of  Spring  Hill,  where  the  strata  are  less  dis- 
turbed than  elsewhere,  the  limestones  present  a  synclinal  fold  whose  axis 
lies  on  tlfe  west  side  of  the  ridge  east  of  Spring  Hill.  Adjoining  the 
Hoosac  fault  lies  a  low,  narrow  ridge  separated  from  the  main  body  of  lime- 
stone by  a  north  and  south  fault,  beyond  which  the  limestones  on  Spring 
Hill  dip  easterly  at  an  angle  of  30°,  the  beds  on  the  opposite  side  of 
the  fold  attaining  angles  as  high  as  60°  westerly.  Measured  on  the  line 
of  the  main  section  there  are  about  3,400  feet  of  limestones  included 


30  GEOLOGY  OF  TI1K  EUREKA  DISTRICT. 

between  the  Hoosac  and  Pinto  faults.  This  entire  series  of  beds  belongs  to 
the  Lower  Coal-measures,  evidence  of  their  age  being  found  in  the  charac- 
teristic fossils  obtained  at  both  the  top  and  the  bottom  of  the  limestones.  Car- 
bon Ridge  possesses  a  simple  structure,  a  single  block  inclined  uniformly  to 
the  east,  the  beds  varying  slightly  from  60°.  Here,  however,  the  position 
of  the  uppermost  beds  of  limestone  is  determined  by  the  overlying  Weber 
conglomerates.  Limestones  form  the  west  base  and  crest  of  the  ridge,  the 
conglomerates  coming  in  all  along  the  east  slope  and  stretching  out  toward 
the  Pinto  fault  until  buried  beneath  the  acidic  pumices  and  tuffs.  The 
limestones  afford  about  the  same  thickness  of  beds  as  developed  on  Spring 
Hill,  and  the  overlying  Weber  conglomerates  measure  1,900  feet,  assum- 
ing a  uniform  dip  and  the  absence  of  all  faulting.  This  series  of  beds 
of  Lower  Coal-measure  limestones  and  Weber  conglomerates  is  similar  to 
the  section  exposed  on  Alpha  Ridge  and  Weber  Peak  in  the  Diamond  Moun- 
tains, the  thickness  being  about  the  same.  It  is  the  sequence  of  strata  most 
commonly  met  with  in  the  Great  Basin  ranges  wherever  we  find  a  broad 
limestone  body  overlain  by  one  of  sandstone. 


TERTIARY    ROCKS. 


Tertiary  Lavas.— Subsequent  to  the  movements  that  folded  and  faulted 
by  powerful  dynamic  forces  this  great  body  of  Paleozoic  strata  came  the 
pouring  out  of  volcanic  lavas,  the  only  other  rocks  that  play  an  important 
part  in  the  geological  history  of  the  Eureka  Mountains.  These  lavas  were 
forced  to  the  surface  not  only  after  the  crumpling  of  the  beds  and  blocking 
out  of  the  mountains,  but  after  very  considerable  erosion  had  carved  the 
deepest  canyons  and  brought  about  the  configuration  of  the  country  much 
as  it  is  seen  to-day.  Evidence  of  this  erosion  before  the  pouring  out  of  the 
lavas  is  shown  by  the  position  of  many  extensive  bodies  of  lava  in  the 
bottoms  of  the  largest  canyons,  and  by  the  blocking  up  of  ancient  drainage 
channels  through  the  welling  out  of  erupted  masses,  necessitating  new  outlets. 
It  is  evident  that  a  very  long  period  of  time  must  have  elapsed  subsequent 
to  the  building  up  of  the  Paleozoic  masses  before  the  breaking  out  of  the 
lavas.  Although  no  direct  evidence  of  the  age  of  these  lavas  can  be  found 
in  the  Eureka  District,  they  are  regarded  as  belonging  to  the  Tertiary 


QI  ATi;i!NAl;V    I  >K POSITS.  31 

period.  In  many  ways  they  bear  the  closest  resemblance  in  their  mode  of 
occurrence,  to  similar  lavns  elsewhere  in  the  Great  Basin,  when;  evidence  of 
their  age  has  been  determined  by  their  relation  to  sedimentary  strata  carry- 
ing a  Miocene  or  Pliocene  fauna  or  flora.  In  mineral  and  chemical  composi- 
tion the  lavas  show  great  variations,  hornblende-andesite,  dacite,  rhyolite, 
pyroxene-andesite,  and  basalt  being  well  represented,  with  a  wide  range  in 
structural  and  physical  features.  A  description  of  these  different  lavas  and 
their  relations  to  each  other,  as  well  as  their  geologic-i  1  relations  to  the 
orographic  blocks,  will  be  found  in  the  chapter  devoted  to  a  discussion  of 
the  Tertiary  rocks. 

QUATERNARY    DEPOSITS. 

Quaternary  Valleys.— The  Eureka  Mountains  rise  out  of  a  broad  plain 
everywhere  covered  by  Quaternary  deposits  that  stretch  away  in  all  direc- 
tions far  beyond  the  limits  of  the  present  survey.  The  atlas  sheets  accom- 
panying this  work  fail  to  indicate  the  relative  area  occupied  by  the  moun- 
tains to  that  of  the  desert  plains,  but  an  extension  of  the  map  only  a  few 
miles  more  on  all  sides  would  at  least  have  shown  how  completely  the 
mountains  were  surrounded  by  a  broad  expanse  of  the  so-called  sage-brush 
deserts.  With  a  single  exception  these  broad  plains  open  one  into  the 
other,  the  only  barrier  being  the  Diamond  Mountains,  which  separate  Dia- 
mond Valley  from  Newark  Valley. 

Newark  Valley  and  Fish  Creek  Basin  are  simply  extensions  of  the 
same  great  plain,  the  former  situated  on  the  east  and  the  latter  on  the  south 
of  the  Eureka  Mountains.  The  Fish  Creek  Basin  connects,  by  means 
of  a  narrow  pass  south  of  the  Fish  Creek  Mountains,  with  Antelope  Valley, 
a  few  miles  beyond  the  limits  of  the  map.  Antelope  Valley  may  be  re- 
garded as  a  southern  extension  of  the  broad,  desert-like  expanse  of  Hayes 
Valley,  which  stretches  far  toward  the  north  on  the  west  side  of  the  Pinon 
Range.  Hayes  Valley  connects  with  Diamond  Valley  by  the  narrow 
gorge  known  as  The  Gate,  which  is  simply  a  low  pass  cut  down  to  the 
level  of  the  plain  through  which  the  former  valley  at  one  time  drained  into 
the  latter. 

Little  time  has  been  devoted  to  the  investigation  of  the  Quaternary 
geology  in  the  immediate  region  of  Eureka,  but  so  far  as  the  deposits  have 


32  GEOLOGY  OF  THE  EUKEKA  DISTRICT. 

been  studied  they  resemble  closely  those  found  in  the  neighboring  valleys, 
and  do  not  offer  much  of  special  or  local  interest. 

During  the  Quaternary  period  vast  accumulations  of  detrital  material 
were  brought  down  from  the  mountains  and  transported  far  out  upon  the 
neighboring  plain  or  laid  down  upon  the  flanks  of  the  outlying  foothills. 
These  deposits  have  been  classed  under  two  distinct  epochs — an  upper  and 
a  lower  Quaternary. 

Lower  Quaternary.— The  earlier  deposits,  or  the  lower  Quaternary,  are 
for  the  most  part  lacustrine,  made  up  of  finely  comminuted  stratified  sands 
and  clays  carrying  varying  amounts  of  calcareous  material.  All  the  beds 
have  a  prevailing  light  yellowish  color.  -  They  form  the  so-called  alkali 
flats  of  Nevada,  and  when  dry  resemble  a  hard  tile  pavement,  but  when 
moist  have  all  the  disagreeable  qualities  of  a  plastic  clay,  well  nigh  impas- 
sable. Nowhere  within  the  neighborhood  of  Eureka  have  they  been  cut  by 
water  channels  for  more  than  a  few  feet,  and  at  the  time  of  our  investigation 
no  deep  borings  for  water  had  been  made.  In  consequence  no  reliable  data 
exist  for  a  correct  estimate  of  their  thickness,  which  in  places  ma}-  reach 
several  hundred  feet.  No  recent  shells  have  as  yet  been  found  in  the  few 
exposures  observed  along  the  stream  beds.  On  the  map  the  line  of  de- 
marcation between  the  upper  and  lower  divisions  of  the  Quaternary  has 
been  drawn  somewhat  arbitrarily,  it  being  by  no  means  easy  to  separate, 
sharply,  the  finer  material  of  the  upper  series  from  the  lacustrine  deposits 
underlying  them. 

Upper  Quaternary.— The  upper  or  mountain  Quaternary  is  made  up  of 
angular  material  varying  in  size  from  large  bowlders  to  fine  sand  and 
gravel.  It  is  in  all  cases  traceable  to  the  neighboring  mountains,  the  nature 
of  the  coarser  fragments  depending  upon  the  rock  exposure  above  it.  The 
material  is  subaerial  in  origin.  It  everywhere  fringes  the  flanks  of  the 
mountains,  encroaching  upon  the  area  of  the  underlying  lacustrine  beds  for 
shorter  or  longer  distances,  according  to  the  configuration  of  the  hill-slopes 
or  the  transporting  power  of  floods  and  freshets.  The  finer  material  is,  nat- 
urally, transported  the  greater  distance,  consequently  it  gradually  becomes 
mingled  with  and  forms  a  superficial  layer  over  the  lower  Quaternary  de- 
posits. South  of  Prospect  Peak  and  opposite  the  entrance  to  Secret  Can- 


UPPER  QUATERNARY.  33 

yon,  these  upper  Quaternary  accumulations  extend  up  the  flanks  of  the 
mountains  for  1,500  feet  above  the  lowest  part  of  Fish  Creek  Valley,  every- 
where concealing-  the  nature  of  the  underlying  rocks. 

Most  of  the  intervening  meridional  valleys  lying  between  the  parallel 
ranges  of  Nevada  consist  of  narrow,  trough-like  depressions,  in  compaii- 
son  with  the  level  plains  bordering  the  Eureka  Mountains.  In  western 
Utah  and  eastern  Nevada  these  valleys  exhibit  great  similarity  as  regards 
their  physical  and  geological  history.  They  have  been  described  at  great 
length  by  Mr.  Clarence  King1  and  Mr.  G.  K.  Gilbert,2  both  of  whom  have  de- 
voted much  time  to  the  study  of  the  Quaternary  accumulations  and  the  cli- 
matic conditions  under  which  the  material  was  laid  down.  Many  local  details 
of  these  valleys  may  also  be  found  in  the  volume  devoted  to  the  descriptive 
geology  of  the  Fortieth  Parallel  Exploration,3  and  the  reader  who  desires 
to  pursue  the  subject  further  is  referred  to  the  works  quoted. 

1  U.  S.  Geol.  Explor.  of  the  Fortieth  Parallel,  vol.  I.     Systematic  Geology. 

2U.  S.  Geol.  Surv.,  Monograph  I.     Lake  Bonneville. 

3  U.  S.  Geol.  Explor.  of  the  Fortieth  Parallel,  vol.  n.     Descriptive  Geology. 


5ION 


CHAPTER    III. 
CAMBRIAN  AND  SILURIAN  ROCKS. 

CAMBRIAN    ROCKS. 

Rocks  of  the  Cambrian  period,  with  the  exception  of  two  small  expos- 
ures, are  confined  to  Prospect  Ridge,  forming  all  the  more  elevated  por- 
tions and  the  steep  slopes  of  both  sidels.  Indeed,  the  ridge  is  almost 
wholly  made  up  of  Cambrian  sedimentary  beds.  Silurian  rocks  perfectly 
conformable  with  the  upper  beds  of  the  Cambrian  come  in  only  along  the 
outlying  spurs  and  foothills  to  the  east  and  north.  All  along  the  east  slope 
of  the  ridge  these  beds  exhibit  a  nearly  uniform  thickness,  but  attain  their 
greatest  development  in  the  region  of  Prospect  Peak,  where  the  lowest 
members  of  the  group  are  best  exposed.  Here  the  Cambrian  rocks  measure 
about  7,700  feet  from  base  to  summit.  They  have  been  divided  into  five 
epochs,  designated  by  local  names,  as  follows:  Prospect  Mountain  quartzite, 
Prospect  Mountain  limestone,  Secret  Canyon  shale,  Hamburg  limestone, 
and  Hamburg  shale.  The  varied  physical  differences  in  the  composition  of 
the  sediments  cause  them  to  fall  readily  into  these  five  epochs,  each  char- 
acterized by  its  own  distinctive  geological  and  topographical  features. 
The  fauna  also  agrees  with  geological  divisions  and  adds  its  own  evidence 
to  strengthen  them.  So  far  as  known,  nowhere  else  in  the  state  of  Nevada 
do  the  Cambrian  rocks  afford  as  fine  geological  sections  as  at  Eureka ;  nor 
have  they  elsewhere  been  subjected  to  as  careful  a  survey.  The  great 
thickness  of  the  group,  the  simplicity  of  structure  in  the  region  of  Prospect 
Peak,  the  slight  rnetamorphism  of  the  strata,  and  the  uniformity  of  dip 
over  wide  areas  and  across  many  thousand  feet  render  a  study  of  the  sedi- 
ments a  comparatively  simple  matter  and  far  easier  than  most  Cambrian 
areas  in  other  regions  of  the  world. 

34 


PROSPECT  MOUNTAIN  QUARTZITE.  35 

Prospect  Mountain  Quartzite.— This  group  lies  at  the  base  of  the  Cambrian 
series  at  Eureka  and  is  consequently  the  oldest  sedimentary  rock  exposed. 
It  takes  its  name  from  the  peak,  the  highest  point  along  the  ridge,  where  it 
reaches  its  broadest  development  and  forms  the  greater  part  of  its  western 
slope.  With  one  or  two  breaks  in  the  continuity,  the  quartzite  may  be 
traced  along  the  base  of  the  ridge  northward  to  Ruby  Hill,  where,  as  the 
footwall  of  the  Richmond  and  Eureka  Consolidated  Mines,  it  becomes 
of  considerable  economic  interest.  There  can  be  no  question  that  the 
quartzite  of  Prospect  Peak  and  that  of  Ruby  Hill  are  identical.  From 
Ruby  Hill  the  quartzite  curves  around  the  end  of  the  mountain,  following 
the  east  side  of  the  ridge,  and  stretches  southward  for  more  than  a  mile 
until  abruptly  lost  by  a  fault.  The  only  occurrence  in  the  district  of  this 
quartzite  is  on  Prospect  Ridge.  On  Prospect  Peak  the  strata  have  a  thick- 
ness of  1,500  feet  and  occur  distinctly  bedded,  but  in  some  localities  all 
lines  of  stratification  appear  to  be  wanting.  At  the  base  of  the  series  the 
beds  are  largely  composed  of  conglomerates  and  brecciated  masses  firmly 
cemented  together  with  ferruginous  material,  with  the  weathered  surfaces 
deeply  stained  by  iron.  In  the  conglomerates  quartz  pebbles  may  occasion- 
ally be  seen,  showing  compression  and  flattening  on  their  broader  sides, 
arranged  in  beds  parallel  to  the  planes  of  stratification.  The  upper  beds 
are  usually  finer  grained,  carrying  less  iron  oxide.  In  the  Charter  Tunnel, 
the  only  locality  where  they  have  been  exposed  by  mining  exploration, 
they  show  highly  metamorphosed  beds  derived  from  impure  siliceous  mate- 
rial. 

Interstratified  throughout  the  quartzite  are  occasional  bands  of  fine 
grained  arenaceous  and  micaceous  shales  only  a  few  feet  in  thickness.  No 
organic  remains  have  been  found  in  this  group,  although  diligent  search 
was  made  in  the  interstratified  shales,  as,  if  they  occur,  they  would  be  of 
the  highest  paleontological  interest,  extending  the  Cambrian  fauna  lower 
than  has  yet  been  known  in  the  Great  Basin.  The  Prospect  Mountain 
quartzite  differs  from  the  Eureka  quartzite,  the  next  overlying  siliceous 
group,  in  being  more  ferruginous  and  in  general  less  uniform  in  texture, 
carrying  throughout  more  or  less  clayey  material,  while  the  latter  quartzite 
is  a  nearly  pure,  highly  altered  sandstone. 


36  GEOLOGY  OF  THE  EUEEKA  DISTRICT. 

Prospect  Mountain  Limestone.— Directly  over  the  Prospect  Mountain  quartz- 
ite  occurs  the  Prospect  Mountain  limestone,  which  forms  the  greater  part  of 
the  ridge  and  both  slopes  of  the  mountain  all  the  way  from  Ruby  Hill 
southwai'd  to  the  entrance  of  Secret  Canyon.  Beyond  the  limits  of  the 
mountain  these  beds  are  unknown  in  the  district.  It  is  difficult  to  define 
sharply  the  characteristic  features  of  this  group,  changes  are  so  frequent  in 
the  deposition  of  the  sediments,  not  only  in  the  vertical,  but  lateral  extension. 
Secondary  alterations  caused  by  the  intrusion  of  eruptive  rocks  and  vari- 
ations in  color  near  the  ore  bodies  tend  to  conceal  the  original  nature  of 
the  rock.  Breccias  firmly  held  together  by  calcite  are  of  common  occur- 
rence, while  throughout  the  group  there  is  abundant  evidence  that  the  beds 
have  been  crushed  and  broken  and  subjected  to  an  enormous  pressure.  In 
general,  however,  the  group  possesses  a  light  bluish  gray  tint  when  observed 
over  large  areas,  although  nearly  all  colors  from  white  to  black  are  found 
in  the  limestone,  which  at  the  same  time  is  characterized  throughout  the 
entire  thickness  of  beds  by  seams  of  calcite  varying  from  one-half  to  6 
inches  in  width,  and  frequently  forming  a  network  of  white  bands. 

In  texture  the  limestone  is  crystalline  and  granular  and  over  wide  areas 
is  so  highly  altered  as  to  obliterate  all  traces  of  organic  life;  and,  while  in, 
places  planes  of  bedding  may  be  distinctly  seen  all  the  way  from  Ruby 
Hill  southward,  they  are  Avholly  wanting  over  the  greater  part  of  the  ridge. 
Stratification  is  well  shown  on  the  seventh  level  of  the  Richmond  Mine  and 
in  the  Eureka  and  Prospect  Mountain  tunnels,  where  the  beds  are  usually 
bluish  gray  in  color. 

Coarse  and  fine  white  marbles,  occasionally  highly  crystalline,  are 
found  on  the  north  end  of  the  mountain,  and  white  and  light  gray  marbles 
more  than  600  feet  in  width  are  cut  by  the  Prospect  Mountain  tunnel, 
good  varieties  being  observed  at  750  feet  and  again  at  1,700  feet  from  the 
entrance  of  the  tunnel.  Analyses  show  them  to  be  nearly  pure  carbonate  of 
lime.  Characteristic  black  limestone  is  found  near  the  Geddes  and  Bertrand 
Mine,  in  Secret  Canyon. 

Numerous  analyses  of  the  rock  from  Ruby  Hill,  Prospect  Mountain 
Tunnel,  and  localities  on  both  sides  of  the  ridge  prove  that  the  beds 
throughout  the  formation  are  a  magnesian  limestone.  Nearly  pure  dolo- 


ANALYSES  OF  LIMESTONE. 


37 


mites  in  thin  layers  have  been  recognized  in  several  localities,  but  the  per- 
centage of  carbonate  of  magnesia  in  most  instances  is  too  low  to  allow  the 
beds,  for  any  considerable  thickness,  to  be  classed  as  dolomite,  neither  is 
there  any  evidence  that  dolomitic  rock  is  characteristic  of  any  particular 
portion  of  this  great  thickness  of  beds.  Both  dolomite  and  pure  limestone 
have  been  shown  to  occur  near  the  large  ore  bodies,  analyses  demonstrat- 
ing, however,  that  there  exists  no  possible  relation  between  the  chemical 
composition  of  the  limestone  and  the  occurrence  of  ore.  Analyses  of  lime- 
stone from  the  neighborhood  of  several  large  ore  bodies  situated  in  widely 
separated  localities  along  the  ridge  and  from  different  geological  horizons 
throughout  the  epoch  give  the  following  results : 


Mine. 

Insoluble 
residue. 

Carbonate 
ofmagnesia. 

1 

0-36 

14-00 

0 

Geddes  &  Bertrand 

13-83 

1-09 

^ 

5-79 

1-84 

4 

0-20 

26-32 

An  analysis  of  the  stratified  limestone  from  the  seventh  level  of  the 
Richmond  mine  may  be  taken  as  a  fair  sample  of  the  limestone  body.  It 
yielded  as  follows: 

Carbonate  of  lime 88-34 

Carbonate  of  magnesia 4-98 

Iron 1-59 

Silica..  4-83 


Total 99-74 

Mr.  Thomas  Price,  of  San  Francisco,  made  a  careful  chemical  study  of 
the  limestones  of  Ruby  Hill,  collecting  his  samples  for  examination  from  the 
most  important  points  on  the  surface  and  from  different  levels  in  the  mines. 
Amono-  the  localities  from  which  the  rocks  were  selected,  were  the  contact 

O 

beds  between  the  limestone  and  the  overlying  Secret  Canyon  shale,  strati- 
fied beds  on  the  seventh  and  eighth  levels  of  the  Richmond  mine,  the  under- 
lying rocks  of  Potts  Chamber,  the  mouth  of  the  Bell  Shaft,  and  near  the  ore 
body  of  the  Tiptop  Incline.  In  sixteen  analyses  the  amount  of  carbonate 


38  GEOLOGY  OF  THE  EUEEKA  DISTEICT. 

of  magnesia  varies  from  1-06  to  44 '35  per  cent;  three  of  them  yielded  less 
than  2  per  cent.  In  nine  out  of  the  sixteen  the  amount  of  the  silica  in  the 
limestone  was  less  than  2  per  cent. 

Many  of  the  beds,  more  especially  the  darker  limestones,  give  evidence 
of  the  presence  of  organic  matter,  even  where  no  signs  of  fossils  are  seen. 
Proof  of  this  is  found  in  the  presence  of  phosphoric  acid  in  the  rock.  Two 
specimens  yielded  O13  per  cent,  evidently  derived  from  the  fossil  remains 
now  almost  wholly  obliterated. 

Sandstone  layers  are  rarely  seen  in  this  group.  Intel-stratified  in  the 
limestone  are  irregular  beds  of  shale,  lenticular  or  wedge-shaped  bodies 
varying  greatly  in  width.  Indeed,  throughout  the  entire  thickness  of  this 
group  they  are  a  characteristic  feature  of  the  beds,  which  pass  by  insensible 
gradations  from  pure  limestone  to  hard  argillaceous  shales.  Occasionally 
they  may  be  traced  interstratified  in  parallel  bands  for  long  distances,  and 
again  the  shale  will  develop  considerable  thickness,  then  rapidly  thin  out 
in  all  directions  For  the  most  part  they  can  be  followed  for  no  great  dis- 
tance. Two  of  these  shale  beds  are  quite  distinctly  marked  on  the  top  of 
the  ridge  to  the  northward  of  Prospect  Peak,  but  all  traces  are  lost  on  the 
surface  to  the  south  of  that  point.  One  of  these  shale  beds  on  the  east  slope, 
however,  attains  so  great  a  thickness  that  it  has  been  designated  Moun- 
tain shale,  to  distinguish  it  from  the  Secret  Canyon  horizon.  Unlike  the 
larger  body  of  overlying  shale  they  are  of  slight  geological  significance,  the 
limestone  both  above  and  below  presenting  nearly  identical  physical  fea- 
tures, and  so  far  as  known  carrying  the  same  organic  forms.  The  Mountain 
shale  comes  to  the  surface  on  the  ridge  near  the  Industry  mine  and  on  the 
steep  slope  of  the  ridge  above  the  Eureka  Tunnel,  across  its  widest  devel- 
opment reaching  over  300  feet  in  thickness.  It  differs  from  the  Secret 
Canyon  shale  in  carrying  alternate  layers  of  argillaceous  and  calcareous 
shales,  the  latter  frequently  passing  into  stratified  shaly  limestone.  This 
body  of  intercalated  shale  presents  some  features  of  economic  interest  bear- 
ing upon  the  ore  deposits,  and  may  possibly  be  the  same  bed  found  in  all 
the  deep  mines  on  Ruby  Hill.  The  thickness  of  the  Prospect  Mountain 
limestone  across  its  broadest  expansion  may  be  taken  at  3,050  feet.  On 
Ruby  Hill,  owing  to  faulting,  it  never  attains  its  full  development. 


HAMBURG  LIMESTONE.  39 

secret  canyon  shale.— The  Prospect  Mountain  limestone  passes  by  gradual 
transition  from  shaly  limestone  into  brown  and  yellow  argillaceous  shales, 
which,  with  the  exception  of  one  or  two  thin  calcareous  layers,  present  a 
very  uniform  character  for  the  entire  distance  from  the  extreme  southern 
end  of  Secret  Canyon,  where  they  first  crop  out,  northward  until  cut  off  by 
a  fault  a  short  distance  northwest  of  the  Eureka  Tunnel.  Toward  the 
upper  portion  of  the  series  the  shale  becomes  gradually  interbedded  with 
thin  layers  of  limestone.  The  designation  of  the  group  is  taken  from  the 
name  of  the  canyon  where  it  appears  most  characteristically  shown.  These 
beds  are  recognized  only  on  Prospect  Mountain  ridge  and  north  of  Ruby 
Hill.  The  topographical  features  of  Prospect  Mountain  are  largely  modi- 
fied by  this  shale  body,  which,  eroding  more  readily  than  either  the  over- 
lying or  underlying  limestone,  has  been  largely  instrumental  in  determining 
the  drainage  channels  of  the  ridge.  There  are  few  finer  examples  of  the 
wealing  away  of  a  soft,  easily  eroded  body  lying  between  two  harder  rock 
masses  than  can  be  seen,  in  Secret  Canyon,  where  the  Prospect  Mountain 
limestone  rises  like  a  wall  on  one  side  and  the  Hamburg  limestone  nearly 
as  abruptly  on  the  other,  while  the  canyon  for  over  3  miles  is  carved  out 
of  the  shale  in  a  deep,  trough-like  valley.  In  their  broadest  development 
the  shale  measures  1,600  feet,  although  in  places  where  they  are  encroached 
upon  by  the  Hamburg  limestone  they  occur  somewhat  thinner.  As  yet  no 
organic  forms  have  been  found  through  the  entire  group,  though  diligent 
search  was  made  for  them  in  the  more  promising  calcareous  layers. 

Hamburg  Limestone.-Transition  beds  of  shaly  limestone,  varying  in  thick- 
ness from  25  to  200  feet,  pass  gradually  into  the  overlying  Hamburg 
limestone,  which  forms  a  prominent,  bold  ridge  between  the  easily  eroded 
overlying  and  underlying  shales,  and,  as  it  is  cut  through  at  regular  inter- 
vals by  east  and  west  drainage  channels,  presents  one  of  the  most  striking 
topographical  features  of  the  region,  and  a  geological  horizon  most  easily 
traced  in  the  field.  On  the  surface  this  limestone  is  dark  gray,  frequently 
grayish  black,  and  throughout  the  greater  part  of  the  thickness  presents  a 
gramilar  texture.  Layers  of  fine  sandstone  and  hard  cherty  bands  occur 
at  irregular  intervals.  In  chemical  composition  it  offers  no  essential  differ- 
ence from  -the  Prospect  Mountain  limestone,  presenting  quite  as  wide  a 


40 


GEOLOGY  OF  THE  EUREKA  DISTRICT. 


range,  both  in  silica  and  magnesia.  Two  complete  analyses  were  made  of 
this  limestone,  one  from  the  summit  and  the  other  from  the  base  of  the 
epoch,  each  representing  a  well  denned  and  persistent  bed,  as  follows: 


Base  of 
Hamburg 
limestone. 

Summit  of 
Hamburg 
limestone. 

Silica      .       .  ...  .  

24-00 

3-94 

Alumina  .         .  .  ..  ..  

•12 

•64 

Ferric  oxide          .  ..  ..  

•12 

•43 

Ferrous  oxide  .  .     

•20 

Manganese                          -.           ... 

•61 

41-97 

51-96 

•80 

•52 

Water 

•16 

•37 

32-62 

40-71 

Phosphoric  acid...  

•07 

•50 

Chlorine           .         .           ......  .  .  .. 

•01 

•01 

Organic  matter 

•03 

Alkalies  .... 

trace 

trace 

Total  

99-87 

99-92 

Aii  examination  made  of  a  dark  compact  limestone  from  the  base  of 
the  Hamburg,  collected  on  the  north  side  of  the  ravine  opposite  the  dump 
of  the  Richmond  shaft,  gave 

Silica -84 

Carbonate  of  magnesia 1-18 

A  gray  dolomite  from  the  350-foot  crosscut  in  the  Dunderburg  mine 
yielded 

Silica -07 

Carbonate  of  magnesia 40-04 

lu  general,  this  limestone  is  sharply  contrasted  in  its  lithological  habit 
with  the  Prospect  Mountain  body,  as  it  is  darker  in  color,  carries  siliceous 
material  in  place  of  the  clayey  beds  of  the  latter,  and  possesses  a  character- 
istic rough  and  ragged  surface  produced  by  weathering.  The  thickness  of 
this  limestone  may  be  taken  at  1,200  feet,  and  except  in  the  shaly  lime- 
stones at  the  top  and  bottom  of  the  series,  110  planes  of  bedding  are  trace- 
able for  any  great  distance.  At  Adams  Hill,  however,  where  the  beds  lie 


CAMBRIAN  FAUNA.  41 

inclined  at  a  much  lower  angle  and  have  undergone  much  less  movement 
and  compression,  stratification  may  be  frequently  observed. 

Hamburg  shale.— This  shale  body  in  general  resembles  the  one  underlying 
the  Hamburg  limestone,  except  that  it  is  by  no  means  as  uniform  in  com- 
position, showing  very  rapid  changes  in  conditions  of  deposition,  becoming 
more  or  less  arenaceous  or  calcareous  throughout  its  entire  thickness  as 
well  as  in  its  lateral  extension  It  is  characterized  by  cherty  nodules,  and 
near  the  top  by  more  or  less  persistent  layers  of  chert  and  sand,  followed 
by  calcareous  shales  which  pass  into  the  overlying  Pogonip  limestone  of 
the  Silurian.  Across  its  broadest  development  it  measures  350  feet,  yet  it 
rarely  maintains  a  uniform  thickness  for  any  long  distance.  The  best 
exposures  are  seen  opposite  the  Hamburg  and  Dunderburg  mines,  and 
again  in  the  •  ravine  north  of  Adams  Hill,  where  it  attains  as  great  a  thick- 
ness as  anywhere  on  the  eastern  slope,  and  is  in  every  way  as  well  shown. 
This  group  is  not  as  thick  as  the  Mountain  shale  in  its  broadest  develop- 
ment in  the  Prospect  Mountain  limestone,  yet  its  persistency,  stratigraphi- 
cal  position,  and  its  relations  to  the  fauna  of  the  Cambrian  render  it  of 
far  greater  importance. 

Cambrian  Fauna.— As  has  already  been  mentioned,  no  evidences  of  organic 
remains  have  been  observed  in  the  Prospect  Mountain  quartzite,  and  the 
conditions  under  which  the  beds  were  deposited  could  hardly  be  considered 
favorable  to  life.  In  the  overlying  Prospect  Mountain  limestone  obscure 
fragments  of  fossils  may  be  detected  at  various  places  throughout  the 
epoch,  but  localities  showing  any  grouping  of  species  or  forms,  sufficiently 
well  preserved  for  identification,  are  limited  to  three  horizons.  The  lower 
of  these  horizons  occurs  at  the  base  of  the  limestone,  in  a  narrow  belt  rest- 
ing on  the  quartzite;  the  second  is  found  in  strata  of  calcareous  shales 
several  hundred  feet  higher  up,  while  the  third  horizon,  which  may  be  two 
or  three  hundred  feet  in  thickness,  lies  at  the  top  of  the  limestones  just 
below  the  Secret  Canyon  shale. 

Directly  overlying  the  quartzite,  in  strata  which  may  be  regarded  as  tran- 
sition beds  between  it  and  the  Prospect  Mountain  limestone,  occur  the  low- 
est organic  forms  obtained  in  the  district,  and  the  equivalent  of  the  lowest 
Cambrian  fossiliferous  strata  in  the  Great  Basin.  Along  the  east  side  of 


42  GEOLOGY  OF  THE  EUEEKA  DISTEICT. 

Prospect  Peak,  near  the  summit  of  the  ridge,  there  may  be  traced  for  over 
a  mile  a  red  arenaceous  and  calcareous  shale,  which  is  lost  to  the  southward, 
but  which,  followed  to  the  northward,  may  be  seen  to  pass  gradually  into  a 
dark  gray  shaly  limestone.  This  arenaceous  shale  may  be  taken  at  100  feet 
in  thickness,  and,  from  the  organic  remains  which  it  carries  and  from  its 
paleontological  and  geological  importance,  has  been  designated  the  OleneUus 
shale.  From  this  horizon  the  following  species  have  been  obtained : 

Kutorgina  prospectensis.  Olenellus  gilberti. 

Ptychoparia  sp.f  Olenellus  iddingsi. 

About  one-half  mile  northward  of  this  locality,  and  in  a  bed  of  lime- 
stone 100  feet  in  thickness,  underlying  the  fossiliferous  arenaceous  shale, 
and,  in  the  same  manner,  resting  directly  upon  the  quartzites,  species  indi- 
cating an  identical  geological  horizon  were  found,  as  follows : 

Olenellus  gilberti.  Olenoides  quadriceps. 

Olenellus  iddingsi.  Scenella  conula. 

Anomocare  parvum. 

These  two  groupings  represent  all  that  have  as  yet  been  identified 
from  this  lower  horizon. 

The  Olenellus  shales  pas?  upward  into  a  great  thickness  of  bluish  gray 
limestone,  with  an  occasional  thin  band  of  interstratified  shale.  The  beds, 
however,  yield  no  well  defined  organic  remains  for  nearly  500  feet,  but  at 
that  horizon  they  furnish  forms  which  might  belong  both  to  the  Olenellus  shales 
below  and  the  next  fossiliferous  strata  above.  Although  localities  yielding 
well  defined  fossils  from  this  second  horizon  are  seldom  met  with,  indistinct 
traces  of  life  are  seen  in  the  limestone  underneath  the  Mountain  shale.  The 
best  known  locality  is  found  at  the  head  of  New  York  Canyon  on  the  long 
sloping  ridge  south  of  the  Fourth  of  July  mine.  Here  were  obtained  the 
following : 

Olenoides  quadriceps.  Agnostus  interstrictus. 

Scenella  conula.  Ptychoparia  prospectensis. 

The  species  of  Ptychoparia  prospectensis  has  not  as  yet  been  found  at 
a  higher  horizon.  Above  this  horizon  the  limestone  is  much  metamorphosed 
and  altered  to  marble,  and  is  so  broken  up  that  well  defined  beds  favorable 


FOSSILS  FROM  THE  EICHMOND  MINE.  43 

to  the  preservation  of  fossils  are  rarely  met  with,  even  the  calcareous  shale 
presenting  but  slight  indications  of  them.  Not  till  within  300  or  400  feet  of 
the  summit  of  Prospect  Mountain  limestone  and  2,000  feet  higher  up  in  the 
strata  was  there  any  grouping  of  fossils  observed.  From  this  horizon,  and 
extending  up  to  the  base  of  the  Secret  Canyon  shale,  numerous  localities 
occur  all  along  the  east  slope  of  Prospect  Mountain,  which  present  a  fauna 
with  much  the  same  grouping  at  each,  and  showing  a  mingling  of  both 
Georgia  and  Potsdam  faunas. 

These  organic  forms  occur  both  in  compact  limestone  and  shaly  calca- 
reous beds,  and  constitute  the  third  and  upper  fossiliferous  strata  of  Prospect 
Mountain  limestone.  The  following  list  contains  most  of  the  species  col- 
lected at  this  horizon  in  New  York  Canyon,  many  of  them  being  found  at 
several  localities : 

Obolella  (like  O.  pretiosa).  Protypus  senectus. 

Lingula  manticula.  Dicelloceplialus  nasutus. 

Agnostus  communis.  Ptychoparia  oweni. 

Agnostus  bidens.  Ptychoparia  occidentals. 

Agnostus  neon.  Ptychoparia  dissiniilis. 
Agnostus  richmondensis. 

From  the  corresponding  beds  in  Secret  Canyon  near  Geddes  and  Ber- 
trand  mine,  and  in  a  compact  black  limestone  a  short  distance  above  the 
base  of  the  Secret  Canyon  shale  belt,  were  collected  the  following  species : 

Kutorgina  whitfleldi.  Aguostus  neon. 

Orthis  eurekensis.  Protypus  expansus. 

Stenotheca  elongata.  Ptychoparia  oweni. 

Agnostus  communis.  Ptychoparia  haguei. 

Agnostus  bidens.  Oleuoides  spinosa. 

In  a  well  denned  stratified  black  limestone  exposed  for  several  hundred 
yards  on  the  seventh  level  of  the  Richmond  mine  were  obtained  the  following 

forms: 

Obolella .  Agnostus  neon. 

Lingula  manticula.  Agnostus  richmondensis. 

Agnostus  communis.  Ptychoparia  oweni. 

Agnostus  bidens. 

The  finding  of  this  grouping  of  fossils  in  the  mine  is  of  some  special 
importance  as  it  adds  paleoiitological  proof  to  structural  evidence  to  show 


44  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

the  geological  age  of  the  limestone  in  which  the  great  bodies  of  ore  upon 
Ruby  Hill  occur. 

The  Prospect  Mountain  limestone  carrying  this  fauna  passes  by  grad- 
ual transition  into  the  Secret  Canyon  shale,  the  passage  beds  being  mainly 
thin  interstratified  layers  of  limestone  and  calcareous  shale.  No  fossils  have 
been  obtained  from  the  argillaceous  strata  of  the  Secret  Canyon  shale 
throughout  its  development,  but  imperfect  fragments  more  or  less  obliter- 
ated have  been  observed  in  several  of  the  more  calcareous  beds.  At  the 
top  of  this  group  the  calcareous  shales  appear,  which  must  be  taken  as 
forming  the  base  of  the  well  known  Hamburg  limestone,  inasmuch  as  they 
indicate  new  conditions  of  sedimentation.  It  is  the  coming  in  of  these  cal- 
careous deposits  that  renders  possible  the  development  and  preservation  of 
a  higher  fauna.  These  calcareous  shales  may  be  recognized  readily  all  along 
the  line  of  contact.  In  places  it  is  well  characterized  by  its  grouping  of 
fossils,  the  same  species  being  observed  from  both  the  east  base  of  Ham- 
burg Ridge  and  the  corresponding  beds  north  of  Ruby  Hill,  presenting  a 
higher  Cambrian  fauna.  The  following  species  have  been  determined  from 

this  horizon : 

Protospongia  fenestrata.  Dicellocephalus  osceola. 

Lingulepis  msera.  Dicellocephalus  richmondensis. 

Lingulepis  minuta.  Ptychoparia  pernasuta. 

Lingula  manticula.  Ptychoparia  laticeps. 

Ipbidea  depressa.  Ptychoparia  bella. 

Acrotreta  gemma.  Ptychoparia  linnarssoni. 

Kutorginr«  minntissima.  Ptychoparia  oweni. 

Hyolithes  priiuordialis.  Ptychoparia  haguei. 

Agnostus  coininunis.  Ptychoparia  similis. 

Agnostus  bidens.  Ptychoparia  unisulcata. 

Agnostus  neon.  Ptychoparia  laeviceps. 

Agnostus  seclusus.  Chariocephalus  tumifrons. 

Dicellocephalus  nasutus.  Ogygia  problematica. 

After  leaving  the  calcareous  shales,  which  form  the  base  of  the  Ham- 
burg limestone,  the  next  fossil  horizon  occurs  in  the  shales  at  the  summit 
of  the  same  group,  and  in  thin  interlaminated  limestones  in  the  overlying 
Hamburg  shale. 


OLENELLCS  SHALE.  45 

This  horizon  has  yielded  the  following  species: 

Lingulepis  inaera.  Dicellocephalus  angnstifrons. 

Lingulepis  miuutu.  Dicellocephalus  inarica. 

Lingula  manticula.  Dicellocepbalus  bilobatus. 

Obolella  discoidea.  Dicellocephalns  oswola. 

Acrotreta  gemma.  Ptychoparia  aflinis. 

Kntorgiua  minutissima.  Ptychoparia  oweni. 

Hyolithes  primordialis.  Ptychoparia  bagnei. 

Aguostus  coinmunis.  Ptychoparia  granulosa. 

Agnostus  bidens.  Ptychoparia  simulata. 

Aguostus  neon.  Ptychoparia  nuisulcata. 

Aguostus  prolongus.  Ptychoparia  breviceps. 

Agnostus  tumidosus.  Arethusina  americana. 

Agnostus  tuimfrons.  Ptychaspis  minitta. 
Dicellocephalus  nasutus. 

The  Olenellus  shales  lie  not  only  at  the  base  of  the  fossiliferous  rocks 
at  Eureka,  but  are  equivalent  to  the  lowest  fossiliferous  strata  as  yet 
recognized  in  the  Great  Basin.  Their  known  stratigraphies!  position 
overlying  the  Prospect  Mountain  quartzite  and  at  the  base  of  a  conform- 
able series  of  limestone  and  shale  of  Cambrian  and  Silurian  age,  measuring 
9,000  feet  in  thickness,  renders  the  question  still  a  matter  of  some  doubt 
whether  older  fossil  bearing  strata  will  ever  be  found  in  Utah  or  Nevada. 
Wherever  the  Olenellus  shale  is  known  to  occur,  it  is  always  found  resting 
upon  siliceous  beds,  and  in  no  single  instance,  where  they  occur -together, 
is  the  thickness  of  the  lower  quartzite  so  great  as  at  Eureka.  Unfortunately 
no  sedimentary  beds  are  known  to  come  to  the  surface  below  the  Prospect 
Mountain  quartzite,  and  of  the  latter  we  are  wholly  ignorant  as  to  its  thick- 
ness. What  is  needed  in  working  out  the  stratigraphy  of  the  Great  Basin 
ranges  is  a  locality  exposing  a  section  of  Lower  Cambrian  rocks  still  lower 
than  those  at  Eureka,  but  at  the  same  time  showing  their  relations  with 
the  Olenellus  shale  and  Prospect  Mountain  limestone  above.  In  the  many 
uplifts  of  quartzose  strata  which  have  been  provisionally  assigned  to  the 
Cambrian  upon  theoretical  grounds,  investigation  may  yet  furnish  proof 
that  certain  iuterstratified  shale  bands  carry  either  a  similar  or  still  lower 
fauna,  and  if  their  structural  relations  with  the  Olenellus  horizon  can  be 
shown,  it  will  make  a  Cambrian  section  much  to  be  desired.  Organic 


46  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

forms  closely  allied  to  the  Olenellus  grouping  of  species  have  been  found 
in  four  places  in  the  Great  Basin:  in  the  Oquirrh  Range,  in  Utah;  in  the 
Highland  and  Timpah-Ute  Ranges,  and  at  Silver  Peak,  in  Nevada.  In  all 
these  they  are  described  as  occurring  in  a  similar  arenaceous  shale  conforma- 
ble to  and  overlying  a  body  of  quartzite,  the  base  of  which  is  not  exposed. 

As  early  as  1874,  Mr.  F.  B.  Meek',  in  a  letter  to  Dr  C.  A.  White,  described 
the  two  species,  Olenellus  gilberti  and  0.  howelli,  from  Pioche,  Nevada.  He 
called  attention  to  the  relationship  existing  between  them  and  Olenellus  ver- 
montana  and  0.  thompsoni,  Hall,  from  the  Georgia  slates  of  Vermont,  and 
to  him  belongs  the  honor  of  first  correlating  these  widely  separated  beds. 

Quite  recently,  after  a  careful  review  of  all  the  material  at  his  com- 
mand, and  a  comparative  examination  in  the  field  of  the  well  known  New 
York,  Vermont,  and  Newfoundland  regions  with  the  more  recently  studied 
Great  Basin  areas  in  Nevada  and  Utah,  Mr.  C.  D.  Walcott'  suggests  dividing 
the  Cambrian  into  three  divisions,  namely :  Lower  Cambrian,  Middle  Cam- 
brian, and  Upper  Cambrian.  These  three  primary  divisions  are  recognized 
in  the  Cambrian  of  Europe,  and  each  of  them  has  received  local  designa- 
tions derived  from  the  name  of  the  region  where  the  terrane  is  typical  and 
well  exposed.  Thus,  in  the  Cordillera,  the  Lower  Cambrian  is  designated 
as  the  Prospect  Mountain  group,  whereas  in  New  York  and  New  England 
it  is  best  known  as  the  Georgia  shale,  from  the  well  known  locality  in  Ver- 
mont. The  Middle  Cambrian  has  as  yet  no  better  typical  locality  than  the 
slates  and  shales  of  St.  John,  New  Brunswick.  The  Upper  Cambrian  is 
usually  spoken  of  as  the  Potsdam  so  well  recognized  all  the  way  from  the 
Atlantic  coast  to  central  Nevada.  At  Eureka  the  latter  epoch  is  represented 
by  the  Hamburg  Ridge. 

Wherever  in  the  Great  Basin,  so  far  as  known  to  the  writer,  the  genus 
Olenellus  has  been  discove  'ed,  the  beds  do  not  attain  a  development  of  more 
than  400  feet;  at  least  they  pass  from  shale  and  shaly  limestone  to  lime- 
stone, in  which  as  yet  no  organic  forms  have  been  recognized.  Only  at 
Eureka  and  in  the  Highland  Range  are  their  structural  relations  with  both 
the  overlying  and  underlying  beds  clearly  made  out.  We  have  very  little 

1  U.  S.  Geographical  Surveys,  West  of  100th  Meridiau,  vol.  iv,  Paleontology,  1877,  p.  47. 
2Stratigraphic  Position    of  the  Olenellus  Fauna  in  North  America  and    Europe.    Am.   Jour. 
Sci.,  3d  ser.,  vol.  xxxvn,  May  and  July,  1889. 


SILURIAN  KOCKS.  47 

knowledge  of  the  structure  at  the  other  localities,  and  in  the  Oquirrh  Range 
the  Olenellus  shales  are  known  to  be  cut  off  by  a  sharp  fault  from  the  Upper 
Cambrian. 

By  reference  to  the  Eureka  section  it  will  be  seen  that  the  Olenellus 
horizon  is  nearly  2,500  feet  below  the  top  of  the  Prospect  Mountain  lime- 
stone, where  there  comes  in  a  fauna  showing  a  mingling  of  Middle  and 
Upper  Cambrian  forms.  At  the  base  of  the  Hamburg  limestone,  1,600  feet 
higher  in  the  strata,  the  true  Potsdam  fauna  of  Wisconsin  and  Minnesota 
is  abundantly  represented  by  a  characteristic  grouping.  By  comparing 
these  lists  of  fossils  from  the  different  horizons,  it  will  be  seen  that  in  this 
group,  at  the  top  of  the  Hamburg  limestone,  there  are  found  seven  species, 
which  first  occur  at  the  top  of  the  Prospect  Mountain  limestone.  They 
pass  up  through  the  beds  at  the  base  of  the  Hamburg  limestone  and,  together 
with  five  additional  species  obtained  for  the  first  time  from  the  latter  hori- 
zon, come  up  to  the  close  of  the  epoch,  making  in  all  twelve  species  common 
to  the  top  and  bottom  of  the  Hamburg  limestone.  Three  species  obtained 
from  both  the  base  and  the  summit  of  the  limestone  are  identical  with 
forms  from  the  Potsdam  sandstone  of  Wisconsin — Hyolitlies  primordiaUs, 
Dicellocephalus  osceola,  Ptychaspis  minuta.  Another,  Lingula  manticula,  first 
described  by  Dr.  C.  A.  White,1  from  the  Schell  Creek  Mountains,  Nevada, 
has  here  at  Eureka  a  wide  range,  extending  from  the  Prospect  Mountain 
limestone  through  the  Hamburg  limestone  and  shale  and  well  up  into  the 
overlying  Pogonip  group  of  the  Silurian. 

SILURIAN   EOCKS. 

Rocks  of  the  Silurian  period  at  Eureka  fall  readily  into  three  epochs. 
From  our  present  knowledge,  it  would  be  a  somewhat  difficult  matter  to 
subdivide  them  still  further,  except  upon  fine  distinctions  founded  upon 
paleontological  grounds,  which  might  not  hold  good  over  any  large  area 
of  country.  These  three  divisions  correspond  with  the  lithological  character 
of  their  sediments,  two  heavy  masses  of  limestone  with  a  sharply  defined 
intervening  bed  of  quartzite.  This  quartzite  is  a  highly  altered  sandstone, 
much  purer  in  composition  than  the  Cambrian  quartzite  below  or  the  sili- 

1 U.  8.  Geographical  Surveys  West  of  the  100th  Meridian,  vol.  iv,  Paleontology,  part  1,  p.  5-'. 


48  GEOLOGY  OF  THE  EUltEKA  DIST1UCT. 

ceous  beds  of  the  Carboniferous  above.  They  have  been  designated  as 
follows:  first,  Pogonip  limestone;  second,  Eureka  quartzite;  third,  Lone 
Mountain  limestone.  The  division  between  the  Cambrian  and  Silurian 
rests  mainly  upon  paleontological  evidences  and  is  by  no  means  a  well 
defined  line  of  separation.  While  the  underlying  Hamburg  shales  of  the 
Cambrian  present  a  lithological  distinction,  the  transition  beds  are  of  vary- 
ing thickness  and  pass  gradually  into  the  overlying  limestone.  Moreover, 
while  at  Eureka  the  argillaceous  shales  serve  to  separate  the  two  periods, 
the  distinction  would  not  hold  good  in  other  regions,  particularly  at  White 
Pine,  where  both  the  Upper  Cambrian  and  Pogonip  are  well  developed, 
with  a  great  thickness  of  strata  and  an  abundant  fauna,  but  without  a  well 
recognized  intermediate  shale  belt.  Wherever  in  the  Great  Basin  the 
Silurian  is  exposed,  conformably  overlying  the  Cambrian,  there  occur  at 
the  same  horizon  a  commingling  of  species  of  both  periods,  but  this  con- 
dition of  things  presents  no  valid  objection  against  the  division  of  any  two 
periods,  for  the  argument  holds  with  equal  force  between  the  limestones  of 
the  Upper  Silurian  and  Devonian,  and  between  the  limestones  of  the  latter 
and  the  Carboniferous. 

Pogonip  Limestone.— The  name  given  to  this  epoch  is  taken  from  Pogonip 
Ridge  at  White  Pine,  and  was  first  employed  by  the  Geological  Exploration 
of  the  Fortieth  Parallel  to  designate  the  great  belt  of  limestone  at  the  base  of 
the  Silurian  period.  At  White  Pine  this  epoch  is  remarkably  well  exposed 
and  of  much  greater  thickness  than  at  Eureka,  although  at  the  latter  locality 
it  covers  large  areas  and  may  be  equally  well  studied,  both  in  its  structural 
relations  and  faunal  development.  On  the  line  of  the  Section  E  F  (atlas 
sheet  xm)  the  transition  between  the  Hamburg  shale  and  Pogonip  passes 
gradually  upward  from  argillites  and  fine  grained  arenaceous  beds  with 
interstratified  calcareous  shales  into  purer  limestones  distinctly  bedded.  The 
limestone  is  for  the  most  part  bluish  gray,  but  near  the  top  is  of  a  darker 
tint,  in  places  becoming  almost  black.  It  is  distinguished  lithologically  from 
both  the  lower  belts  of  limestone  in  its  more  massive  bedding,  fineness  of 
texture,  and  the  smoothness  of  its  weathered  surfaces.  This  last  feature, 
however,  holds  true  only  in  a  broad,  general  way,  as  bands  of  chert  fre- 
quently produce  roughness  of  texture  resembling  Hamburg  limestone. 


THICKNESS   OF  POGONIP   LIMESTONE.  49 

Chemistry  shows  no  characteristic  difference  between  this  limestone  and 
the  older  masses,  the  beds  being  more  or  less  magnesian  throughout  their 
entire  vertical  range.  A  complete  analysis  was  made  of  a  siliceous  variety, 
taken  from  near  Wood  Cone,  yielding  the  following  result : 

Silica 9-345 

Alumina 0-309 

Ferric  oxide 0-289 

Ferrous  oxide 

Manganese 

Lime 50-011 

Magnesia : 0-535 

Water 0-130 

Carbonic  acid 39-111 

Phosphoric  acid 0-240 

Chlorine 0-030 

Organic  matter Traces. 

Alkalies Traces. 


Total 100-000 

To  the  east  of  Jackson  Mine,  where  the  beds  are  well  exposed  and  lie 
inclined  at  a  nearly  uniform  angle,  they  measure  2,700  feet  across  their 
greatest  development.  This  thickness  is  probably  surpassed  by  the  beds 
on  the  long  spur  southwest  of  Wood  Cone,  but  there  they  stand  nearly  ver- 
tical, in  some  places  dipping  eastward  and  in  others  westward,  occasionally 
showing  evidences  of  faulting,  which  prevents  any  reliable  estimate  of  their 
thickness.  It  is  probable  they  measure  over  3,000  feet.  An  estimate  of  the 
strata  at  White  Pine  gives  over  5,000  feet  of  limestone.  At  first  sight  it  would 
appear  as  if  there  must  have  been  some  displacement  of  beds  along  Pros- 
pect Mountain,  but  the  succession  of  a  rich  fauna  with  the  same  character- 
istic specific  forms  at  the  base  and  summit  of  the  epoch  at  both  Eureka  and 
White  Pine  would  preclude  such  a  supposition  and  the  simplicity  and  uni- 
formity of  structure  go  to  show  that  such  is  not  the  case. 

Fauna  of  the  Pogonip  Epoch.— Throughout  the  entire  thickness  of  the  Pogo- 
nip  beds,  organic  remains  characterize  the  epoch.  At  the  base  there  is  a 
decided  mingling  of  species,  a  number  of  Potsdam  forms  extending  up- 
ward for  some  distance  into  the  limestone.  Passing  upward,  however, 
these  species  gradually  diminish  and  there  comes  in  rapidly  a  numerous 
fauna  representing  higher  and  higher  forms,  till  midway  in  the  beds  nearly 
MON  xx 4 


50  GEOLOGY  OF  THE  EUEEKA  DISTRICT. 

all  the  characteristic  Cambrian  fauna  have  passed  away  and  genera  equiv- 
alent to  the  Chazy  horizon  of  New  York  have  taken  their  place,  and  near 
the  top  a  grouping  of  fossils  comes  in  strongly  indicating  the  Trenton  hori- 
zon. In  the  collections  made  from  the  Pogonip  beds  at  Eureka,  nearly 
eighty  species  have  been  determined,  a  large  proportion  of  them  forms 
found  for  the  first  time  either  at  Eureka  or  White  Pine,  while  many  of 
them  are  common  to  both  localities  and  from  the  same  stratigraphies!  posi- 
tion in  the  beds.  Many  of  them  are  identical  with  species  found  in  New 
York  and  Canada  and  along  the  Atlantic  border. 

Fifteen  species  comprise  all  those  forms  which  have  been  recognized 
as  common  to  both  the  Cambrian  period,  and  Pogonip  epoch  of  the  Silu- 
rian, and  several  of  these  present  a  wide  vertical  range  extending  downward 
to  the  summit  of  the  Prospect  Mountain  limestone. 

The  list  is  as  follows: 

Lingnlepis  maera.  Agnostus  neon. 

Lingulepis  minuta.  Ptychoparia  affinis. 

Lingula  manticula.  Ptychoparia  oweni. 

Obolella  discoidea.  Ptychoparia  granulosus. 

Acrotreta  gemma.  Ptychoparia  haguei. 

Leptajna  melita.  Ptychoparia  unisulcatus. 

Agnostus  conununis.  Arethusina  americaua. 
Agnostus  bidens. 

Only  two  species  of  the  genus  Dicellocephalus  have  been  recognized  as 
yet  in  the  Pogonip  group  at  Eureka,  D.  finalis  and  D.  inexpectans,  both  new 
to  science.  They  occur  associated  together  several  hundred  feet  above  the 
base,  at  a  horizon  where  many  of  the  Cambrian  species  have  already  dis- 
appeared. Of  the  genus  Dicellocephalus  only  two  species  are  known  from 
the  corresponding  beds  at  White  Pine.  Near  the  base  of  the  Pogonip  in  a 
limestone  northeast  of  Adams  Hill,  a  decided  mingling  of  both  Cambrian 
and  Silurian  occur,  as  seen  by  the  following  list: 

Lingulepis  maera.  Agnostus  neon. 

Obolella  discoidea.  Ptychoparia  (Euloma)  affinis. 

Acrotreta  gemma.  Ptychoparia  oweni. 

Leptsena  melita.  Ptychoparia  haguei. 

Triplesia  calcifera.  Ptychoparia  unisulcatus. 

Hyolithes  vauuxemi.  Illsenurus  eurekeusis, 
Asaphu.s  caribouensis. 


FAUNA   OF  THE   POGONIP.  51 

A  number  of  localities  southeast  of  Ruby  Hill  represent,  in  their  fauna, 
a  somewhat  higher  horizon,  the  most  favorable  for  collecting  being  found 
on  the  first  ridge  southeast  of  the  Jackson  Mine,  where  the  base  of  the 
Pogonip  beds  are  wanting,  having  been  cut  off  by  the  Jackson  fault. 
These  beds  yielded  the  following  species  : 

Lingulepis  msera.  Ptychoparia  (Enloma)  affinis. 

Lingnla  manticula.  Arethusina  americaim. 

Acrotreta  gemma.  JJlaeuurus  eurekensis. 

Leptoeua  melita.  Asaphus  caribouensis. 

Orthis  hamburgensis.  Asaphus  (sp.  undt.). 

Directly  east  of  the  Hamburg  Ridge  ,and  several  hundred  feet  above 
the  last  locality,  a  grouping  of  fossils  comes  in  which  is  characteristic 
of  a  slightly  higher  horizon  : 

Lingulepis  ina-va.  Triplesia  calcifera. 

Lingula  mauticula.  Tellinomya!  hamburgensis. 

Disciua  (sp.  undt.).  Dicellocephalus  finalis. 

Acrotreta  gemma.  Dicellocephalus  inexpectans. 

Schizambon  typicalis.  Ptychoparia  annectans. 

Obolella  ambigua.  Ptychoparia  oweni. 

Orthis  hamburgensis.  Amphiou  (sp.  undt.). 
Orthis  testudinaria. 

This  horizon  may  be  easily  identified  by  collections  of  fossils  more  or 
less  complete  from  numerous  other  localities  in  the  district.  From  about 
this  point  in  the  limestone  the  older  persistent  forms  gradually  disappear, 
and  the  new  species  introduced  in  the  above  list  become  more  and  more 
abundant,  as  is  evidenced  by  the  increasing  number  of  localities  where  they 
occur  as  higher  strata  are  reached. 

In  a  compact  gray  limestone  southwest  of  McCoy's  Ridge  are  the  fol- 
lowing : 

Orthis  perveta.  Plumulites  (sp.  undt.). 

Orthis  testudinaria.  Ceraurus  (sp.  undt.). 

Triplesia  calcifera.  Elcenurus  eurekensis. 

Maclurea  annulata.  '  Asaphus  caribouensis. 

Midway  in  the  Pogonip,  the  genera  Meceptaculites,  CJuefcff*,  rimroto- 
maria,  Maclurea,  Bathyurus,  Asaphus,  and  Cyphaspis,  make  a  decided  change 


52  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

in  the  fauna  from  the  Hamburg  limestone.  Many  of  these  genera  gradually 
give  way  and  are  replaced  by  others,  until  at  about  800  or  1,000  feet  below 
the  summit  the  fauna!  development  is  shown  by  a  grouping  of  fossils  made 
at  two  widely  separated  areas,  which  begin  to  foreshadow  the  strongly 
marked  fauna  at  the  summit  of  the  epoch.  From  the  east  slope  of  the 
ridge  east  of  the  Hamburg  Ridge  there  were  collected — 

Beeeptacolites  ellipticus.  Maclurea  amiulata. 

Cystidian  plates.  •  Bellerophon  ? 

Orthis  perveta.  Ortlioceras  (like  O.  mnlticauieratnm). 

Triplesia  calcifera.  Cypliaspis  brevimarginatus. 

Raphistoma?  Ilkeruirus  eurekeiisis. 

Pleurotomaria  loueusis.  Asaphus?  curiosus. 

And  from  the  long,  eastern  slope  of  White  Mountain,  about  800  feet 
from  the  top  of  the  mountain  and  probably  nearly  the  same  distance  below 
the  summit  of  the  Pogonip,  there  were  collected  as  follows  : 

Monticulopra.  Pleurotomaria  lonensis. 

Orthis  testudiuaria.  Endoceras  proteiforme. 

Raphistoma  nasoiii.  Ortlioceras  sp.  ? 

Maclurea  annulata.  Bathyurus  similis. 

Maclurea  subaimulata.  Asaphus  cariboueDsis. 

Throughout  the  upper  600  feet  of  the  Pogonip,  wherever  organic 
remains  have  been  observed,  the  association  of  genera  are  much  the  same, 
the  horizon  being  well  determined  both  by  the  fauna  and  the  position  of 
the  overlying  Eureka  quartzite.  In  many  areas  where  the  Eureka  quartzite 
forms  the  surface  rock  an  underlying  limestone  several  hundred  feet  in 
thickness  is  frequently  exposed,  which  carries  paleontological  evidences  of 
the  upper  Pogonip  strata.  Two  localities  in  these  upper  Pogonip  beds 
have  furnished  a  rich  and  varied  fauna.  From  a  dark  limestone  on  the 
summit  of  White  Mountain  the  following  species  have  been  determined : 

Receptaculites  ellipticus.  Tellinomya  contraeta. 

Receptaculites  elongatus.  Helicotoma  sp? 

Receptaculites  mammillaris.  Orthoceras  inulticaineratum. 

Cystidean  plates.  Eudoceras  (like  E.  inultitubulatum). 

Strophomena  uemea.  Leperditia  bivia. 

Orthis  perveta.  Leperditia  sp? 

Orthis  testudiuaria.  Beyrichia  sp? 


FAUNA  OF   THE  POGONIP.  53 

A  similar  grouping  of  fossils  was  procured  in  the  Fish  Creek  Moun- 
tains a  short  distance  below  the  quartzite,  numerous  localities  yielding 
nearly  identical  lists: 

Receptacotites  ellipticus.  Modiolopsis  occidens. 

Receptaculites  elongatus.  Modiolopsis  pogouipeiisis. 

Beceptacnlites  mammillaris.  Pleurotomaria  sp? 

Cystidean  plates.  Maclurea  sp  ? 

Ptilodictya-  sp?  Orthoceras multicameratum. 

Monticulopora  sp  ?  Endoceras  protciforme. 

Ortliis  perveta.  Aini)hioii  nevadensis. 

Tellinouiya  contracta.  Ceraurus  sp? 

On  the  north  slope  of  Surprise  Peak,  just  below  the  quartzite,  the  lime- 
stone supplied  the  following : 

Receptaculites  mainmillaris.  Rapliistoma  nasoui. 

Cystidean  plates.  Pleurotomaria  ? 

Ortliis  perveta.  Maclurea  annulata. 

Ortliis  tricenaria.  Leperditia  bivia. 

A  convenient  locality  to  those  visiting  Eureka  and  wishing  to  exam- 
ine the  Upper  Pogonip  beds  may  be  found  on  the  west  side  of  Caribou 
Hill,  which  has  furnished  a  few  typical  forms  : 

Ortliis  perveta.  Receptaculites  maminillaris. 

Ortliis  tricenaria.  Maclurea  auimlata. 

Asaphus  cariboueusis. 

Other  localities  which  have  presented  evidences  of  the  same  horizon 
may  be  found  in  Goodwin  Canyon,  at  the  head  of  Lamoureux  Canyon, 
and  in  the  limestones  not  far  from  the  line  of  the  general  section  E  F, 
atlas  sheet  xm. 

This  grouping  of  fossils  from  the  summit  of  the  Pogonip  limestone  is 
of  special  interest  on  account  of  the  commingling  of  species  and  the  position 
of  the  strata.  Ascending  in  the  beds  it  will  be  found  that  the  Cambrian 
fauna  entirely  disappears,  the  life  of  the  Middle  Pogonip  gradually 
passes  away,  and  new  species  come  in  until  the  grouping  of  the  fauna 
presents  an  aspect  peculiarly  its  own.  Two  species  of  the  genus  Modiolop- 
sis, and  the  characteristic  fossil,  Tettinomya  contracta,  foreshadow  still  higher 
strata,  indicating  the  coming  in  of  the  Trenton  horizon.  The  summit  of 
the  Pogonip  is  also  marked  by  an  increase  in  the  number  of  species  of 


54  GEOLOGY  OF  THE  EUREKA  DISTRICT. 


fri/rnaria,  0.  t<-xti<tH>i(U-iK,  and  0.  perveta,  characteristic  forms  in  K 
York  and  Wisconsin.  A  marked  feature  of  this  upper  horizon  is  the  pres- 
ence of  the  genus  Ecccptaculites,  three  species  having  been  identified.  Im- 
mense numbers  of  specimens  of  one  of  them,  R.  mammiUaris,  are  found 
throughout  the  beds  with  a  vertical  range  of  several  hundred  feet,  and  are 
abundant  where  all  other  fossils  are  wanting.  Oraptolites,  in  the  Pogouip 
epoch  at  Eureka,  are  represented  by  a  single  undetermined  species,  which, 
according  to  Mr.  C.  D.  Walcott,  resembles  closely  G.  bijid/ix. 

Eureka  Quartzite.—  The  name  of  the  district  has  been  employed  to  desig- 
nate this  formation,  as  during  the  progress  of  the  survey  the  quartzite  was 
determined  for  the  first  time  as  a  distinct  geological  epoch  and  its  strati- 
graphical  position  clearly  defined.  Up  to  this  time  the  occurrence  of  a 
broad  belt  of  quartzite  lying  between  two  massive  bodies  of  Silurian  lime- 
stone had  never  been  recognized.  Moreover,  nowhere  else  in  the  Great 
Basin  has  the  formation  been  so  carefully  studied.  It  lies  superimposed 
directly  on  the  Pogonip  limestone,  and  where  the  upper  beds  of  the  latter 
epoch  are  exposed  they  are  frequently  capped  by  a  greater  or  less  thickness 
of  the  quartzite,  as  is  well  shown  on  Caribou  Hill  and  McCoy's  Ridge. 
Again,  the  position  of  the  Eureka  quartzite  is  clearly  brought  out  by  the 
patches  of  quartzite  left  by  erosion  upon  the  massive  Pogonip  beds  of  Fish 
Creek  Mountains.  No  horizon  is  more  marked  in  its  physical  features 
than  the  Eureka  quartzite.  Besides  its  frequent  occurrence  as  a  capping 
rock,  its  snow-white  color,  and  its  tendency  to  fracture  in  mural-like  escarp- 
ments render  it  easily  recognizable  wherever  it  occurs. 

The  Eureka  quartzite  is  made  up  almost  entirely  of  siliceous  grains  firmly 
compacted  together.  It  possesses  a  granular  texture  and  a  vitreous  luster, 
and  for  the  most  part  is  free  from  partings  parallel  to  the  planes  of  bedding. 
At  the  base  of  the  formation  the  quartzite  is  colored  red  and  gray  by  iron, 
but  it  rapidly  passes  into  white,  with  an  occasional  bluish  or  purplish  tinge, 
frequently  presenting  a  mottled  coloring.  In  general  it  is  exceptionally  free 
from  seams  or  patches  of  ferruginous  material,  its  purity  and  uniformity  of 
composition  and  marble-like  appearance  being  a  marked  feature  of  the  hor- 
izon. In  one  or  two  places  it  shows  a  brecciated  appearance,  with  fine, 
cherty  masses,  notably  on  Hoosac  Mountain.  In  the  neighborhood  of 


EUREKA  QUARTZITE.  55 

McCoy's  Ridge  it  has  been  quarried  for  fluxing  purposes  at  the  smelting 
furnaces,  the  rock  yielding  nearly  two  dollars  in  gold  per  ton,  which  paid 
for  hauling.  Whether  the  gold  is  of  primary  origin  in  the  quartzite  or 
whether  it  was  derived  from  some  vent  carrying  mineral  matter  in  solution 
has  never  been  determined.  The  locality  where  the  rock  was  quarried  is 
situated  near  the  Hoosac  fault,  and  in  close  proximity  to  ore  bodies. 

The  i-idge  extending  southwest  from  Castle  Mountain  shows  a  fine 
body  of  the  Eureka  quartzite,  the  southern  escarpment  of  which  exposes  a 
section  300  feet  in  thickness.  Numerous  specimens  collected  at  intervals 
across  the  quartzite  were  subjected  to  microscopic  examination.  All  the 
upper  portion  of  the  rock  proved  to  be  an  exceptionally  pure  and  fine 
quartz,  the  grains  averaging  between  0-02  and  0'03  millimeters  in  size, 
with  a  granitoid  structure ;  that  is,  the  grains  did  not  show  rounded 
outlines,  but  instead  presented  irregular  shapes  that  fitted  into  each  other 
and  firmly  crystallized  together  without  fine  groundmass  between  them. 

The  quartzite  is  free  from  impurities  but  full  of  fluid  inclusions  with 
moving  bubbles,  some  of  them  evidently  liquid  carbonic  acid.  The  minute 
fluid  cavities  appear  white  in  incident  light.  An  examination  of  the  quartzite 
indicated  that  the  entire  rockmass  had  undergone  a  recrystallization  of 
the  material  and  was  not  by  any  means  a  simple  solidification  and  packing 
together  of  quartz  grains.  In  other  words,  it  is  a  true  quartzite  and  not  a 
compact  sandstone,  hardened  by  superincumbent  rock.  Even  under  the 
microscope  the  rock  appears  to  cany  but  little  oxide  of  iron.  Toward 
the  upper  part  of  the  formation  the  microscope  detects  increasing  numbers 
of  needles  and  grains  of  iron  oxide,  accounting  for  the  change  of  color 
both  in  the  unaltered  rock  and  on  the  weathered  surfaces  of  the  larger 
detached  blocks.  Particles  of  calcite  also  begin  to  appear  some  distance 
beneath  the  Lone  Mountain  limestone,  associated  with  the  quartz  grains, 
while  at  the  base  of  the  quartzite  there  is  a  very  decided  increase  in  the 
amount  of  lime  present. 

Although  not  differing  materially  from  those  observed  elsewhere,  the 
most  satisfactory  section  across  the  quartzite  was  made  just  west  of  Castle 
Mountain.  Here  the  quartzite  presents  a  perpendicular  cliff,  300  feet  in 
thickness,  resting  horizontally  on  the  Pogonip  Ihnestone.  The  subjoined 


56 


GEOLOGY  OF  THE  EUREKA  DISTRICT. 


section  is  numbered  from  the  top  downward,  the  numbers  inclosed  in 
brackets  coinciding  witli  the  specimen  number  in  the  collection.  Through- 
out the  section  the  quartzite  is  for  the  most  part  vitreous  without  partings 
parallel  to  the  bedding,  the  coloring,  however,  being  in  nearly  horizontal 
planes,  passing  insensibly  from  one  tint  to  another. 


•0 

e 

a 
(§ 

3 

1 

o 

10 
10 

30 
10 

31) 

Id 
20 

10 
20 

to 
so 

10 
20 
20 

40 

:v               NO. 

=¥                    No. 
V 

igr    i.: 

J7              No. 
-T-            No. 
\3             No. 

I 

2 

3 
4 

5 

6 

7 
7« 
8 
9 

10 
11 

lib 
12 

13 

14 
15 

(391) 
(390) 

(389) 
(388) 
(387)  ) 
(386)  \ 
(385) 
(384)  ) 
(383)  ] 
(382) 
(381) 
(380)  ) 
(379)  ( 
(378)  ) 
(377)  [ 
(376)  } 
(375) 

(374) 

(373) 
(372) 

No. 

No. 

9           No. 

No. 

No. 

r    NO. 

.-'*         No. 
it 
No 

== 

13 

,4         No. 

No. 

IS 

10  feet  of  white  vitreous  quartzite. 

10  feet  grayish  white,  with  segregation  of  fer- 
ruginous material. 

30  feet  white  and  vitreous. 

10  feet  purple  and  white,  vitreous. 

30  feet  purplish  white,  with  three  narrow  bauds 
of  dark  gray  granular  quartzite. 

10  feet  dark  gray  quartzite. 

20  feet  white,  banded  with  steel  grayj  dark  gray 
quartzite  bands  in  No.  7. 

20  feet  dark  gray  and  white,  banded  and  mottled. 

20  feet  light  gray,  fine  granular. 

40  feet  white  and  pinkish  white. 

60  feet  dark  gray  passing  into  light  gray,  with 
bands  more  or  less  calcareous,  weathering 
red. 

20  feet  gray,  having  cross  bedding  brought  out 
by  weathering. 

20  feet  dark  steel  gray  quartzite,  somewhat  cal- 
careous. 

20  feet  siliceous  limestone.  )      Pogonip 

40  feet  black  compact  limestone.  )     limestone. 


360  feet 

Fio.l.—  Eureka  quartzite  west 
of  Castle  Mountain. 


The  junction  between  the  quartzite  and  the  underlying  limestone  pre- 
sents a  sharp  line  of  demarcation  and  indicates  an  abrupt  change  in  the 
deposition  of  sediments. 

Although  the  Eureka  quartzite  is  probably  not  more  than  a  few  hun- 
dred feet  in  thickness,  it  can  be  estimated  only  approximately,  as  an  uncon- 
formity exists  between  it  and  the  next  overlying  group.  Over  the  large 
area  covered  by  the  exposures  of  the  quartzite,  evidences  of  denudation 
prior  to  the  deposition  of  the  Lone  Mountain  limestone  may  be  observed  in 
the  mountains  connecting  the  Fish  Creek  Range  with  Prospect  Ridge,  but 
no  satisfactory  estimate  of  the  amount  seems  possible.  Again,  not  only 


LONE  MOUNTAIN  LIMESTONE.  57 

different  horizons  of  the  Lone  Mountain  limestone,  but  even  of  the  Devon- 
ian, are  seen  to  repose  directly  upon  and  to  overlap  the  quartzite.  Under 
any  circumstances  the  quartzite  would  be  difficult  to  measure,  inasmuch  as 
over  the  greater  part  of  the  area  stratification  lines  are  wanting,  and  the 
beds  are  frequently  broken  up  by  a  succession  of  small  parallel  faults  not 
always  easy  to  recognize,  rendering  the  amount  of  displacement  still  more 
difficult  to  estimate.  These  minor  displacements,  when  the  rocks  lie  nearly 
horizontal,  produce  steps  and  mural  faces  wherever  the  quartzite  occurs  as 
the  surface  rock.  In  nearly  all  such  instances  the  Pogonip  beds  are  exposed 
in  the  more  deeply  eroded  canyons.  On  the  other  hand,  where  the  beds  are 
inclined  at  high  angles,  accompanied  by  numerous  faults,  the  formation  fre- 
quently presents  the  appearance  of  a  much  greater  thickness  than  is  really 
the  case,  as  is  seen  on  Hoosac  and  Lookout  mountains. 

The  best  estimates  place  the  thickness  of  the  beds  at  about  500  feet, 
although  no  escarpment  of  the  quartzite  free  from  faulting  presents  quite 
so  broad  a  development.  No  fossils  have  been  obtained  from  this  horizon, 
nor  is  it  likely  that  they  will  be  found.  The  microscope  shows  clearly  how 
complete  an  alteration  has  taken  place  since  the  original  sand  deposits  were 
laid  down,  so  that  all  traces  of  fossils,  if  any  existed,  must  have  been 
obliterated. 

Lone  Mountain  Limestone. — Next  above  the  Eureka,  quartzite  comes  a  body  of 
limestone  without  any  transition  beds,  the  change  in  the  character  of  depos- 
its being  unusually  abrupt.  The  designation  of  the  epoch  is  taken  from  a 
bold  isolated  mountain  which  rises  out  of  the  plain  a  few  miles  to  the  north- 
west of  the  Eureka  District,  where  it  is  seen  in  its  full  development  better 
than  in  the  immediate  area  of  the  map.  Not  only  is  it  well  shown  at  Lone 
Mountain,  but  in  a  continuous  section  its  relations  are  clearly  made  out  with 
the  other  members  of  the  Silurian  period  and  with  the  overlying  body  of 
Devonian  limestone.  The  section  at  Lone  Mountain  is  given  in  detail  at 
the  end  of  this  chapter. 

The  Lone  Mountain  epoch  may  be  divided  upon  paleontological 
grounds  into  two  horizons,  which,  for  convenience,  are  provisionally  desig- 
nated as  the  Trenton  and  Niagara.  The  lowest  beds  resting  immediately  on 
the  quartzite  are  a  steel-gray,  almost  black,  gritty  limestone,  in  most  places 


58  GEOLOGY  OF  THE  EUBEKA  DISTRICT. 

without  traces  of  bedding,  and  so  altered  as  to  have  obliterated  all  evidences 
of  organic  remains.  Ascending  the  strata  these  steel-gray  beds  pass  up 
into  dark  bluish  gray  limestone,  which  in  one  locality  north  of  Wood  Cone 
yielded  a  small  lot  of  fragmentary  and  poorly  preserved  fossils,  but  which 
represent  a  characteristic  Trenton  grouping.  These  black  and  gritty  beds 
are  recognized  in  but  few  places  at  Eureka,  mainly  in  the  southwest  corner 
of  the  district,  along  the  southern  base  of  the  Mahogany  Hills.  It  is  quite 
possible  that  the  horizon  covers  a  larger  area  than  has  been  supposed,  but 
if  such  is  the  case  the  beds  have  undergone  so  great  a  lithological  change 
that  their  recognition  seems  impossible  without  paleontological  evidence, 
and  that  is  wholly  wanting.  Moreover,  the  beds  resting  upon  the  quartzite 
in  other  places  resemble  higher  strata  in  the  Lone  Mountain  epoch. 

This  limestone  appears  to  be  magnesian  throughout ;  a  siliceous  variety 
from  the  fossiliferous  beds  north  of  Wood  Cone  yielded  8'41  per  cent  silica 
and  2'55  per  cent  magnesium  carbonate.  The  thickness  of  these  lower 
beds,  in  which  the  Trenton  aspect  of  the  fauna  is  so  strongly  marked,  may 
be  taken  at  300  feet,  at  least  the  black  and  blue  limestone  presents  about 
that  development  before  passing  into  the  upper  strata. 

Above  the  horizon  with  the  Trenton  grouping  the  rocks  pass  gradually 
into  light  gray  siliceous  limestone,  with  a  peculiar  saccharoidal  texture,  in 
places  becoming  almost  white  and  wholly  without  bedding.  On  the  surface 
the  limestones  weather  brown  and  buff,  their  light  colors  throughout  a 
great  vertical  range  standing  out  in  strong  contrast  with  the  other  massive 
limestone  beds  of  the  Paleozoic.  It  weathers  in  rounded  outlines,  breaking 
with  an  irregular  fracture  and  presenting  a  monotonous  appearance  weari- 
some to  the  eye.  Rock  of  this  character  makes  up  by  far  the  greater  part 
of  the  horizon,  and  then  by  slow,  imperceptible  changes  it  becomes  darker 
in  color,  with  more  and  more  tendency  to  develop  planes  of  stratification, 
and  gradually  passes  into  the  overlying  limestone  of  the  Devonian. 

As  already  mentioned,  an  unconformity  exists  between  the  Eureka, 
quartzite  and  the  Lone  Mountain  limestone.  There  is  therefore  no  direct 
evidence  in  the  district  of  the  thickness  of  the  limestone.  The  average 
thickness  of  strata  exposed  has  been  taken  at  1,800  feet,  but  it  is  probable 
that  this  is  under  rather  than  over  estimated,  and  at  Lone  Mountain  they 


TRENTON    FAUNA.  59 

attain  a  somewhat  greater  development,  at  least  2,000  feet  being  exposed. 
In  most  localities  at  Eureka  where  the  limestone  rests  upon  the  quartzite 
the  upper  members  of  the  epoch  are  wanting,  and  in  others  they  pass  under 
the  Devonian  without  any  means  of  measuring  their  thickness.  Another 
difficulty  arises  from  the  impossibility,  on  our  present  knowledge,  of  de- 
termining a  line  of  separation  between  the  Silurian  and  Devonian,  as  no  sharp 
lithological  distinctions  exist  and  there  is  no  means  of  telling  exactly  how 
far  down  in  the  limestone  a  Devonian  fauna  comes  in.  It  is  known,  how- 
ever, that  Silurian  corals  extend  up  into  the  limestone  about  1,500  feet  from 
the  base,  and  the  dark  blue  limestone  which  characterizes  the  Devonian 
makes  its  appearance  about  300  feet  higher  up  in  the  series. 

Fauna  of  the  Lone  Mountain  Limestone.— The    fauna    obtained    from  the    Lone 

Mountain  limestone,  although  meager  and  most  of  the  material  too  poorly 
preserved  for  specific  ^identification,  is  of  special  interest,  as  it  occupies  a 
most  important  position  in  the  development  of  life  in  the  geological  record. 
Not  only  are  organic  forms  poorly  represented,  but  the  beds  themselves  over 
large  areas  of  the  Great  Basin  have  not  as  yet  been  recognized  and  over 
other  areas  are  known  to  be  wanting.  The  collection  indentifying  the 
Trenton  fauna  was  found  on  a  low  ridge  a  short  distance  northeast  of  Wood 
Cone.  The  list  comprises  several  characteristic  species :  Leptcena  sericea, 
Orthis  subqmdrata,  0.  (like  0.  plicatella),  Trinucleus  concentricus,  and  Asaphus 
platycephalus,  and  representatives  of  the  following  genera :  Streptelasnia, 
Rhynchonella,  Orthoceras,  Cyrtoceras,  Ceraurus,  Dalmanites,  and  Ulanus.  It 
is  worthy  of  special  mention  that  in  this  small  but  representative  collection, 
all  the  more  typical  forms  found  in  the  beds  immediately  below  the  Eureka 
quartzite,  which  indicated  the  coming  in  of  higher  horizons,  are  wanting  or 
at  least  have  not  as  yet  been  found. 

Above  the  Trenton  no  good  grouping  of  fossils  has  as  yet  been  dis- 
covered until  the  Devonian  rocks  are  reached.  The  upper  portion  of  the 
Silurian  limestone  presents  a  most  forbidding  aspect  for  the  preservation  of 
organic  remains,  and  although  diligent  search  was  made  throughout  the 
horizon  it  was  rewarded  only  by  finding  a  few  imperfect  corals,  belonging 
to  the  species  Hall/site*  catrmilatus,  which  is  so  characteristic  of  the  Niagara 
of  the  East,  and  here  found  in  what  should  be  its  true  geological  position. 


60  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

They  have  a  wide  range  and  occur  nearly  1,500  feet  above  the  summit  of 
the  Eureka  quartzite.  The  same  coral  has  been  obtained  from  Lone  Moun- 
tain and  White  Pine,  and  in  both  these  latter  localities  associated  with  the 
genus  Zaphrentis. 

Lone  Mountain.— This  isolated  mass  rises  abruptly  out  of  the  broad  plain 
lying  between  the  Wahweah  and  Pifion  ranges  and  about  15  miles  north- 
west of  the  Eureka  Mountains,  which  shut  in  the  plain  to  the  south- 
west. Its  isolation,  its  great  altitude  as  compared  with  the  length  of  the 
uplift  in  strong  contrast  with  the  neighboring  ranges,  and  its  steep  slope  to 
the  eastward  make  the  mountain  a  most  conspicuous  object.  In  its  geolog- 
ical structure  the  mountain  appears  to  be  a  monoclinal  ridge  of  great  sim- 
plicity and  uniformity,  remarkably  free  from  any  great  faults  and  folds  and 
presenting  a  block  of  strata  about  4,000  feet  in  thickness  and  reaching-  an 
altitude  nearly  2,000  feet  above  the  plain.  The  beds  have  all  the  appear- 
ance of  being  cut  off  by  a  sharp  fault  at  the  south  end  of  the  block,  evi- 
dence of  which  may  be  found  in  the  body  of  Carboniferous  limestone  rest- 
ing against  the  Devonian  at  the  southeast  base  of  the  uplifted  mass.  The 
dip  of  the  strata  upon  Lone  Mountain  is  uniformly  to  the  east  at  an  angle 
of  30°  to  50°,  with  a  strike  a  little  east  of  north.  To  the  geologist  a 
series  of  beds  like  this  at  Lone  Mountain  would  at  all  times  command 
attention,  but  in  this  exposure  of  4,000  feet  of  strata  is  represented  a  sec- 
tion of  the  Paleozoic  rocks  rarely  seen  in  the  Great  Basin  and  so  far  as 
known  nowhere  else  so  well  shown  as  here.  The  value  of  the  exposure 
consists  in  the  simplicity  with  which  the  three  divisions  of  the  Silurian  are 
brought  out  in  the  same  continuous  section.  At  the  western  base  of  the 
mountain  the  upper  members  of  the  Pogonip  come  to  the  surface,  but. 
with  an  exposure  of  only  about  375  feet  of  beds.  Within  this  belt,  how- 
ever, a  fauna  strikingly  characteristic  of  this  horizon  is  found  and  almost 
identical  with  that  occurring  in  the  corresponding  Pogonip  beds  at  Eureka. 
A  few  hours'  search  yielded  the  following: 

Receptaculites  mammillaris.  Modiolopsi.s  occidens. 

Monticulopora  sp.  ?  Modiolopsis  pogouipeiisis. 

Cystidian  plates.  Hellicotoina? 

Acrotreta  (like  A.  subconica).  Plenrotomaria  loneusis. 

Stropboineua  neraea.  Murchisonia  sp.  I 


SECTION  ACJKOSS  LONE  MOUNTAIN.  (H 

Orthis  lonensis.  Maclurea  annulate. 

Orthis  perveta.  Maclurea  carinata. 

Orthis  tcstudiiiiiria.  Maclurea  sp.? 

Streptorhynchus  minor.  Cyrtolites  sinuatus. 

Coleoprion  minuta.  llla-nus  sp.? 

Resting  upon  the  Pogonip  comes  the  Eureka  quartzite,  but  with  less 
thickness  than  the  corresponding  beds  at  Eureka.  Immediately  above  the 
quartzite,  with  but  little  development  of  transition  beds,  occur  the  light 
colored  siliceous  limestones,  measuring  at  least  2,000  feet.  These  beds  form 
the  greater  part  of  the  western  slope  of  the  mountain,  and  are  so  character- 
istically shown  as  to  make  the  local  name  of  Lone  Mountain  an  appropriate 
one  to  designate  the  epoch.  In  the  lower  limestones,  resting  directly  upon 
the  quartzite,  the  Trenton  fauna  appears  to  be  wanting,  and  it  is  by  no 
means  certain  that  the  beds  are  represented.  At  all  events  the  bluish  gray 
limestone  characteristic  of  the  Trenton  at  Eureka  and  White  Pine  has  not 
been  recognized.  On  the  other  hand,  throughout  the  entire  epoch  evi- 
dences of  organic  remains  are  exceedingly  meager  and  confined  to  silicified 
corals  imperfectly  preserved.  The  Niagara  coral,  Halysites  catemilatus,  which 
usually  occurs  several  hundred  feet  above,  is  found  here  within  50  feet 
of  the  quartzite. 

The  light  colored  siliceous  limestone  passes  up  gradually  into  the  dis- 
tinctly bedded  Nevada  limestone  of  the  Devonian,  which  forms  the  summit 
of  the  ridge,  and  as  the  strata  dip  eastward  make  up  the  greater  part  of  the 
eastern  slope.  It  is  by  no  means  certain,  however,  that  a  displacement  of 
strata  does  not  extend  along  the  eastern  face  of  the  uplifted  mass,  the  base 
of  the  ridge  not  having  been  examined. 

Mr.  C.  D.  Walcott  made  the  following  section  across  Lone  Mountain 
(see  Fig.  2): 

Feet. 

1.  Dark   gray  limestone,  with  brown  and  variegated  layers  iuterbedded. 

Typical  Devonian  fauna.     (Nevada  limestone.) 1, 500 

2.  Siliceous  bluish  gray  limestone  breaking  up  into  shaly  bands  carrying 

abundant  fossils  of  the  Lower  Devonian.     (Nevada  limestone.) 200 

3.  Siliceous  limestone,  light  brown,  gray,  and  buft'  in  color,  with  Hull/site* 

catenulatitit  near  the  base;  passing  up  into  beds  almost  white,  with  blue 
and  gray  tints,  followed  by  alternating  dark  and  light  beds.  (Lone 
Mountain  limestone.) 


62 


GEOLOGY  OF  THE  EUEEKA  DISTRICT. 


4.  White  quartzite.     (Eureka  quartzite.) 

5.  Dark  gray  limestone,  massive  bedding,  with  intercalated  slialy  layers 

carrying  a  typical  Silurian  fauna.     (Pogonip  limestone.) 

6.  Siliceous  cherty  limastoiie 


Feet 

200 


300 

75 


4,275 


^Pogonip 
Limestone. 


'F.ureKa.  JuoneM1.  .Devonian. 

Quartzite  Limestone  Limestone 

FIG.  2.  —  Section  across  Lone  Mountain. 


Iii  the  Nevada  limestone  at  Lone  Mountain  the  fauna  is  exceedingly 
rich  in  species.  A  list  of  the  fossils  occurring  here,  together  Avith  some 
remarks  upon  their  geological  significance,  will  be  found  in  the  following 
chapter  in  the  discussion  of  the  Devonian  rocks. 


CHAPTER   IV. 
DEVONIAN  AND  CARBONIFEROUS  ROCKS. 

DEVONIAN   ROCKS. 

By  imperceptible  gradations  limestones  of  the  Lone  Mountain  epoch 
pass  upward  into  those  of  the  Devonian  period,  and  as  no  definite  horizon 
separating  them  has  as  yet  been  determined  110  accurate  measurements  of 
their  respective  thicknesses  can  be  given.  Devonian  rocks  cover  a  far 
greater  area  in  the  district  than  those  of  any  other  period;  they  are  much 
more  widely  distributed  and  present  a  thickness  greater  than  either  the 
Cambrian  or  Silurian.  In  no  part  of  the  Great  Basin  are  they  better 
exposed  than  at  Eureka,  and  as  nowhere  else  have  they  been  so  carefully 
investigated  the  district  must  long  remain  a  typical  one  for  the  study  of 
Devonian  strata.  Notwithstanding  the  beds  present  a  rich  fauna,  only  two 
subdivisions  of  the  Devonian  have  been  made — first,  Nevada  limestone,  and 
second,  White  Pine  shale — although  taken  together  they  have  a  thickness 
of  about  8,000  feet,  This  division  is  based  upon  a  marked  change  in  both 
the  fauna  and  character  of  the  sedimentation. 

Nevada  limestone.— The  name  selected  to  designate  this  horizon  is  taken 
from  the  name  of  the  state  where  the  epoch  is  so  well  represented  by  a 
broad  development  of  beds  and  the  only  state  or  territory  in  the  Great 
Basin  where  it  has  been  recognized  as  attaining  any  great  thickness  and  its 
limits  and  geological  relations  studied.  As  the  designation  of  the  epoch 
would  suggest,  the  beds  throughout  the  entire  series  are  composed  mainly 
of  limestone,  although  intercalated  beds  of  shale,  quartzite,  and  sandstone 
occur.  The  Lone  Mountain  and  Nevada  limestones  taken  together  present 
an  immense  thickness  of  beds,  lying  between  the  Eureka  quartzite  and 
White  Pine  shale.  Together  they  measure  about  7,800  feet  in  their  broad- 
est development.  The  division  into  Silurian  and  Devonian  is  based  mainly 
upon  paleontological  grounds.  The  transition  in  sedimentation  from  char- 
acteristic Silurian  to  unmistakable  Devonian  is  so  imperceptible  that  a 

63 


64  <;EOLOGY  OF  THE  EUKEKA  DISTRICT. 

boundary  between  them  is  impossible  to  establish,  and,  as  is  usually  the 
case  where  beds  form  a  continuous,  conformable  limestone  series,  a  line 
of  separation  based  upon  faunal  changes  must  always  remain  more  or  less 
arbitrary.  Lithologically,  in  their  broader  features,  the  Silurian  and 
Devonian  limestones  are  quite  distinct;  it  is  only  in  the  intermediate  beds 
that  no  line  can  be  drawn.  The  light  gray  and  white  siliceous  beds  that 
form  the  mass  of  the  Lone  Mountain  present  a  wide  vertical  range,  and 
in  these  beds  are  occasionally  seen  obscure  impressions  of  Niagara  corals, 
and  in  other  localities,  in  similar  rocks  not  much  higher  up  in  the  series, 
occur  Atrypa  reticularis  and  other  forms  foreshadowing  the  Devonian.  It 
is  known  that  characteristic  Lone  Mountain  beds  carrying  Hah/sites 
catenulatus  extend  for  nearly  1,500  feet  above  the  Eureka  quartzite,  and 
that  beds  easily  identified  by  their  organic  remains  bring  the  Devonian 
down  to  about  6,000  feet  below  the  summit  of  the  great  limestone  belt 
lying  between  the  Eureka  quai-tzite  and  White  Pine  shale.  Hatysites  and 
Atrypa  reticularis  were  never  found  associated  together,  although  it  can  not 
be  definitely  stated  that  the  former  fossil  does  not  appear  as  low  down  in 
the  limestone  as  the  highest  occurrences  of  the  characteristic  coral. 

The  Nevada  limestone  presents  broad  elevated  rock-masses  character- 
ized by  bold  escarpments  and  castellated  summits.  Profound  orographic 
movements  have  broken  this  great  body  of  limestone  into  massive  blocks 
intersected  by  gorges  and  canyons,  affording  a  mountain  scenery  both 
grand  and  picturesque,  and  one  rarely  equaled  in  any  limestone  region  of 
the  Great  Basin.  Although  these  uplifted  blocks  afford  abundant  geological 
exposures  across  the  greater  part  of  the  limestone,  in  no  one  instance  is 
there  a  complete  or  in  every  way  satisfactory  section  from  base  to  summit. 
In  many  localities  the  exposures  extend  upward  from  the  summit  of  the 
Lone  Mountain  several  thousand  feet  into  the  Nevada  beds;  in  others  the 
strata  are  well  shown  from  the  top  down  till  cut  off  by  some  line  of  faulting 
which  hides  all  the  lower  limestones.  Frequently  the  lower  beds  of  the 
Devonian  are  buried  beneath  the  Quaternary  plain.  The  region,  how- 
ever, affords  many  excellent  and  overlapping  sections  exposing  from  4,000 
to  5,000  feet  of  rock;  one  continuous  series  of  beds  being  estimated  at  5,400 
feet,  which  includes  nearly  the  entire  Nevada  epoch.  Throughout  the 


NEVADA   LIMESTONE.  65 

Nevada  limestone,  the  physical  features  of  sedimentation  are  sufficiently 
characteristic  to  correlate  the  strata  when  comparing  a  large  number  of 
sections  across  several  thousand  feet,  although  the  details  across  any  one 
section  are  not  persistent  enough  to  determine  with  precision  the  horizons 
over  any  extended  area.  Modoc  Peak,  Combs  Mountain,  Atrypa  Peak, 
Woodpeckers  Peak,  and  Newark  Mountain  afford  typical  sections. 

In  general  the  lower  limestones  are  indistinctly  bedded,  light  gray  in 
color,  and  highly  crystalline,  passing  up  into  brown,  reddish  brown,  and 
gray  beds,  which  are  distinctly  stratified  and  finely  banded  and  striped, 
presenting  a  somewhat  variegated  appearance  on  the  weathered  surfaces. 
This  latter  feature  is  very  persistent  throughout  the  middle  portion  of 
the  limestone.  In  the  upper  members  the  limestones  are  more  massive, 
usually  well  bedded,  and  possess  a  normal  bluish  black  and  bluish 
gray  color.  In  broad  masses  it  is  difficult  to  distinguish  the  upper 
members  of  the  Nevada  limestone  from  the  Carboniferous  limestone, 
and  they  closely  resemble  the  great  bodies  of  the  Wasatch  limestone 
of  Utah.  The  intercalated  bands  of  argillaceous  shale  and  quartzite 
vary  greatly  in  width,  but  do  not  especially  mark  any  part  of  the 
limestone,  except  that  they  occur  more  frequently  in  the  middle  portion 
than  elsewhere.  Calcareous  shales  are  found  throughout  the  epoch.  The 
limestones  are  everywhere  more  or  less  magnesian,  nearly  pure  dolomites 
frequently  occurring  in  narrow  layers.  At  the  base  of  the  section  north  of 
Modoc  Peak  (Fig.  3)  the  rock  carries  4O62  per  cent  of  magnesium 
carbonate,  with  Ol  per  cent  of  insoluble  residue.  In  band  15,  of  the  same 
section,  the  dark  colored  limestone  carries  T26  per  cent  of  carbonate  of 
magnesia,  while  the  light  colored  rock  holds  26  78  per  cent. 

The  Modoc  Section.— A  section  in  detail  across  the  strata,  extending  from 
the  summit  of  the  Nevada  limestones  nearly  to  the  base,  was  made 
by  Mr.  J.  P.  Iddings.  It  was  constructed  across  the  high  ridge  lying 
between  Signal  and  Modoc  peaks,  beginning  with  the  lowest  rocks  exposed 
at  a  point  northwest  of  the  latter  peak  just  east  of  the  Modoc  fault,  and 
terminating  at  the  eastern  base  of  the  hills  where  the  uppermost  beds  pass 
beneath  the  valley  accumulations  (atlas  sheet  vn).  The  section  measures 
5,400feet.  The  beds  trend  obliquely  across  the  ridge,  striking  N.  50°-55°  W. 
MON  xx 5 


66 


GEOLOGY  OF  THE  EUKEKA  DISTRICT. 

Nevada  limestone — Devonian. 


17 

i6 

IS 


MODOC   SECTION. 


(Dark  gray  to  bluish  black  massive  limestone  poor  in  fossils ;  quite  well 
bedded ;  weathering  partly  smooth  and  dark  colored ;  partly  rough 
and  pitted  and  of  lighter  color ;  mostly  compact  and  massive,  also  of 
uneven  texture ;  with  numerous  calci  te  seams. 


(  Light  and  dark  colored  limestone  with  Stromatopora  and   Chcetetes; 
ISO    )     contains  two  layers  thinly  bedded  (fissile). 
so       Compact  light  yellow  sandstone. 

Light  and  dark  colored  limestone  in  layers  10  to  20  feet  thick,  with 
Stromatopora  and  Clustetes. 


Dark  colored  limestone  with  Stromatopora  and  CJiaitetes. 


t  Alternating  layers  (about  10  feet  thick)  of  dark  and  light  gray  lime- 
900  \     stone,  finely  banded  and  lined ;  weathering  brownish  gray ;  in  places 
(     bearing  Chatetes. 


so      Compact  yellow  sandstone. 

ISO     Dark  and  light  gray  limestone;  indistinct  bedding. 

jo     (  Compact  yellow  sandstone. 

)  Dark  and  light  colored  limestone  interbedded  in  layers  from  4  to  10 
250  feet  thick. 

270     Light  gray  siliceous  limestone ;  very  siliceous  near  base. 

<  Alternating  beds  of  dark  and  light  gray  limestone;  at  base  30  feet; 
ISO  j  very  siliceous  limestone ;  with  cross  bedding  on  weathered  surface. 
•3O  Compact  yellow  sandstone. 

f.   I  Dark  and  light  gray  limestone  in  thick  belts  of  dark,  lighter,  and 
"*•      \     gray  colors. 


22.5    Dark  dense  limestone ;  well  bedded;  bearing  fossils. 
iOO    Shaly  limestone  rich  in  fossils. 


(  Light  gray  siliceous  limestone,  with  fine  lines  of  bedding;  in  upper 
SSO  \     portion    weathering  in  almost  rectangular   fragments;    growing 
(     less  siliceous  toward  the  bottom. 


/4O     Light  gray  highly  crystalline,  saccharoid  dolomite ;  not  siliceous. 


2400 

FIG.  3.— Nevada  limestone— Modoc 
section. 


LAMOUKEUX  SECTION.  67 

The  Lamoureux  Section. — The  section  along  the  limestone  ridge  northeast 
of  the  head  of  Lamoureux  Canyon  (atlas  sheet  ix)  exposes  4,300  feet  of 
strata,  the  lowest  members  resting  immediately  upon  the  Eureka  quartzite  of 
the  flat-top  hill  about  three-quarters  of  a  mile  south  of  Atrypa  Peak.  It  is 
impossible  to  say  just  how  great  a  thickness  of  these  beds  should  be 
assigned  to  the  Lone  Mountain  epoch.  Unquestionajbly  the  lower  members 
of  the  Silurian  are  wanting,  and  if  a  line  be  drawn  placing  the  alternating 
blue  and  light  gray  bedded  rocks  No.  6,  in  the  Devonian,  it  would  give 
about  800  feet  to  the  lower  group.  About  500  feet  above  this  line  a  fossil- 
iferous  belt  comes  in,  carrying  a  well  known  Devonian  fauna.  This  fossil- 
iferous  belt  may  be  traced  around  to  the  east  slope  of  Atrypa  Peak,  where 
a  most  abundant  fauna  occurs  rich  in  generic  and  specific  forms.  Here  at 
Atrypa  Peak,  however,  there  are  nearly  2,000  feet  of  strata  below  the  fos- 
siliferous  belt  as  against  1,300  feet  in  the  Lamoureux  Section  before  reaching 
the  Eureka  quartzite,  but  as  the  inclination  of  the  beds  can  not  well  be  deter- 
mined no  accurate  measurement  of  the  thickness  can  be  given.  Apparently 
the  lowest  horizon  at  Atrypa  Peak  is  below  the  one  shown  in  the  section, 
although  the  character  of  the  sedimentation  is  much  the  same. 

The  section  is  as  follows : 

Section  Hast  of  Lamoureux  Canyon — 4,300  feet. 

Feet. 

1.  Brown  and  blue  limestone,  well  bedded,  with  occasional 

mottled  beds 300 

2.  Brownish  gray,  finely  striped,  well  bedded  limestone,  with 

corals 1, 000 

3.  Dark  blue,  light  gray,  and  brownish  limestone 1, 000 

4.  Alternating  dark  and  light  limestone 500 

5.  Fossiliferous  shaly  belt 200 

6.  Light  blue  and  gray  bedded  limestone 500 

7.  Light  colored  siliceous  limestone,  with  indistinct  bedding. .  800 

g    (  Thin  layer  of  black  siliceous  limestone. 
'   (  Eureka  quartzite. 

4,300 

County  Peak  Section. — On  the  east  side  of  the  Eureka  District,  in  the  region 
of  County  Peak,  the  Devonian  rocks  offer  still  another  section  quite  similar 
in  the  character  of  its  sedimentation  to  those  already  given.  It  includes  a 
portion  of  the  Lone  Mountain  rocks  exposed  in  the  bluffs  on  the  east  side 


68  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

of  C.  C.  Canyon  and  extends  eastward  until  the  upper  members  of  the 
Nevada  limestone  are  submerged  beneath  the  great  basalt  flow  of  Basalt 
Peak  and  the  Strahlenberg. 

County  Peak  Section — 5,200  feet. 

Feet. 

1.  Evenly  bedded,  bluish  gray  limestone,  with   interbedded 

bauds  of  dark  limestone 600 

2.  Irregularly  bedded,  blue  limestone,  with  intercalated  seams 

of  quartzite 1, 600 

3.  Yellowish  gray  quartzite,  with  narrow  bauds  of  gray  silice- 

ous limestone 100 

4.  Massive  beds  of  siliceous  limestone  alternating  with  beds  of 

pure  gray  limestone  and  narrow  bands  of  quartzite 700 

5.  Massive,  gray  vitreous  sandstone 100 

6.  Siliceous  limestone  in  massive  beds  more  or  less  siliceous  in 

thin  bands,  carrying  shaly  limestone  belts 800 

7.  Grayish  white,  vitreous  sandstone 100 

8.  Gray  and  blue  limestone  well  bedded 500 

9.  Light  colored,  compact  quartzite  changing  from  red  to  white  50 
10.  Massive,  light  colored  limestone  without  bedding,  more  or 

less  siliceous . .  650 


5,200 

In  this  section  the  lower  700  feet  are  assumed  to  belong  to  the  Lone 
Mountain,  giving  4,500  to  the  DeAronian.  This  leaves  about  1,500  feet  of 
the  Upper  Devonian  strata  wanting  as  compared  with  the  beds  in  the  region 
of  Modoc  Peak.  These  upper  beds  are  again  well  shown  at  Newark  Moun- 
tain and  Mahogany  Hills. 

white  pine  shale.— Conformably  overlying  the  Nevada  limestone  occurs  a 
heavy  body  of  black  shale,  which  has  been  designated  as  above,  it  having 
been  first  recognized  as  a  distinct  horizon  in  the  White  Pine  mining  district 
to  the  southeast  of  Eureka.  It  occupies  a  clearly  defined  stratigraphic 
position  with  a  marked  change  in  the  character  of  sedimentation  and  a 
fauna  distinct  from  both  the  underlying  and  overlying  horizons. 

There  are  only  two  large  bodies  of  White  Pine  shale  at  Eureka,  but 
they  both  offer  excellent  rock  exposures,  one  west  of  Newark  Mountain, 
the  other  east  of  Sentinel  Peak.  The  shale  is  best  studied  west  of  Newark 
Mountain  (atlas  sheet  vi),  where  it  forms  the  entire  rock  mass  through  which 


DEVONIAN   PLANT  EEMAINS.  69 

Hayes  Canyon  has  been  eroded  and  where  its  geological  relations  with  the 
Nevada  limestone  below  and  the  Diamond  Peak  quartzite  above  may  be 
easily  recognized.  The  shale  attains  its  greatest  development  east  of 
Sentinel  Peak  and  Sugar  Loaf,  but  as  it  is  cut  off  from  the  Nevada  lime- 
stone by  a  north  and  south  fault  which  passes  up  Rescue  Canyon  its 
stratigraphical  relations  with  the  underlying  strata  are  not  as  clearly  shown 
as  at  the  first  locality,  while  the  overlying  beds  are  buried  beneath  the 
detritus  of  the  plain.  The  thickness  across  the  broadest  part  of  the  White 
Pine  shale  east  of  Sugar  Loaf  may  be  placed  at  2,000  feet.  A  marked 
feature  of  the  beds  is  the  rapid  changes  which  they  undergo,  both  in  their 
lateral  and  vertical  extension,  passing  abruptly  from  pure,  argillaceous,  black 
shale  into  beds  more  or  less  arenaceous  and  frequently  carrying  interca- 
lated beds  of  red,  friable  sandstone  appearing  as  lenticular  masses  in  the 
shale.  In  Hayes  Canyon  the  beds  for  the  most  part  are  brownish  black 
shale,  with  thin  bands  of  red  sandstone  while  opposite  Sugar  Loaf  the  inter- 
calated red  sandstone  strata  occasionally  attain  a  thickness  of  100  feet.  Out 
in  the  valley  the  lines  between  the  shale  and  sandstone  may  be  easily  fol- 
lowed for  long  distances,  the  former  occupying  shallow,  trough-like  depres- 
sions and  the  latter  low  intervening  ridges  slightly  elevated  above  the  gen- 
eral level.  Cross  sections  made  at  no  great  distances  apart  differ  widely 
in  the  character  of  the  sediments.  All  evidence  indicates  a  shallow-water 
deposit.  The  formations  at  Eureka  and  White  Pine  are  identical  in  every- 
way except  in  thickness  of  deposits,  at  the  latter  locality  measuring  not 
more  than  600  feet. 

Plant  Remains  in  white  pine  shale.— Impressions  of  plants  which  are  exceed- 
ingly rare  in  Paleozoic  rocks  of  the  Great  Basin  are  very  abundant  and 
form  a  distinctive  feature  of  this  epoch,  notwithstanding  that  everything 
which  has  been  collected  is  of  fragmentary  nature.  The  most  promising 
specimens  for  identification  were  submitted  to  Sir  J.  William  Dawson, 
who,  in  his  report,  called  attention  to  the  poor  state  of  preservation  of  the 
plants.  Under  date  of  Montreal,  June  11,  1889,  he  writes: 

One  slab  contains  a  small  ribbed  stem  referable  to  Goeppert's  Anarthrocanna, 
a  doubtful  Calamitean  plant.  The  specimen  is  not  unlike  those  found  at  Perry,  in 
Maine,  and  Bay  de  Chaleur.  On  the  large  slab  is  also  a  slender  branch  stem  which  I 
suppose  may  be  the  stipe  of  a  fern,  and  from  its  character  and  angle  of  ramification 


70  GEOLOGY  OF  THE  EUKEKA  DISTRICT. 

probably  belongs  to  the  genus  Aneimites,  but  no  trace  of  the  pinnae  can  be  seen.  The 
evidence,  so  far  as  it  goes,  would  indicate  the  Upper  Devonian  (or  Brian,  as  I  prefer 
to  call  it,)  rather  than  the  Middle  Devonian  or  the  Lower  Carboniferous. 

It  will  be  seen  that  this  determination  as  to  the  age  of  the  plants  is 
quite  in  accord  with  the  geological  position  of  the  beds  above  the  Nevada 
limestone  of  the  Devonian  and  directly  below  the  Diamond  Peak  quartzite 
of  the  Carboniferous. 

Notwithstanding  the  great  development  of  the  black  shales  they  have 
as  yet  been  recognized  only  in  the  two  localities  already  mentioned,  Eureka 
and  White  Pine.  On  the  east  side  of  the  Eureka  District,  if  they  are  repre- 
sented at  all,  it  is  only  by  100  feet  more  or  less  of  dark  shaly  beds,  highly 
arenaceous,  and  passing  into  sandstones  and  quartzites  of  the  Diamond 
Peak  beds.  There  seems  to  be  no  doubt  that  the  Diamond  Peak  forma- 
tion in  the  Pinon  Range  rests  conformably  upon  the  Nevada  limestone, 
without  the  interposition  of  any  great  thickness  of  White  Pine  shales, 
although  there  are  a  few  black  sandstones  and  narrow  chert  bands  which 
apparently  represent  the  intervening  argillaceous  epoch.  The  evidence  in 
favor  of  this  correlation  is  strengthened  by  the  presence  of  poorly  preserved 
fragments  of  vegetable  life  wherever  the  black  belt  comes  in.  These  inter- 
vening beds  have  yielded  one  single  species,  Discina  minuta,  which,  accord- 
ing to  Mr.  C.  D.  Walcott,  corresponds  closely  with  typical  specimens  from 
the  Marcellus  shale  of  New  York.  The  fact  that  the  White  Pine  shales  are 
Avanting  over  large  areas,  where  both  the  Devonian  and  Carboniferous  are 
found  together,  renders  it  highly  probable  that  these  shallow  water  deposits, 
although  developed  to  a  great  thickness,  form  exceptional  occurrences, 
and  that  the  Nevada  limestone  passes  over  abruptly  into  sandstones  of 
Carboniferous  age.  On  the  map  (atlas  sheet  v)  these  intervening  beds  on 
both  sides  of  The  Gate  are  included  in  the  Nevada  limestone. 

Fauna  of  the  Devonian.  — As  already  mentioned,  no  subdivisions  in  the  Nevada 
limestone  have  been  made.  Geology  as  yet  fails  to  furnish  sufficient  evi- 
dence for  drawing  any  sharp  demarcation,  sedimentation  having  gone  on  too 
uniformly  under  similar  conditions  to  form  any  marked  change  in  the  char- 
acter of  the  beds.  From  the  sections  already  given  it  will  be  seen  that  this 
epoch  was  essentially  a  limestone-making  one,  the  amount  of  sandstone  de- 


DEVONIAN    FAUNA.  71 

posited  being  relatively  small.  Paleontology  fails  equally  with  geology  to 
point  out  any  strong  reasons  for  subdivisions ;  moreover,  it  would  be  impos- 
sible, from  our  present  knowledge,  to  subdivide  the  epoch  into  horizons  as 
recognized  in  the  Mississippi  Valley  and  the  Appalachians  of  the  Atlantic 
coast.  The  groupings  of  fossils  at  the  base  and  those  at  the  top  show  very 
considerable  difference  in  the  fauna,  but  the  mingling  of  species  throughout 
the  beds  has  rendered  it  difficult  to  draw  any  line  of  separation.  Many  of 
the  species  characteristic  of  a  restricted  horizon  elsewhere  have  been  identi- 
fied in  the  Nevada  limestone,  but  with  a  wide  vertical  range,  and  in  some 
instances  have  reversed  their  relative  positions,  as  recognized  in  New  York 
state.  At  no  distant  day,  when  the  epoch  becomes  still  better  known  and 
comparative  studies  have  been  made  with  other  localities  in  the  Great  Basin, 
it  may  be  quite  possible  and  even  desirable  that  such  divisions  should  be 
drawn.  At  present,  however,  it  will  be  quite  sufficient  to  speak  in  general 
terms  of  an  upper  and  a  lower  horizon. 

The  Nevada  limestone  has  yielded  an  exceedingly  rich  and  well  preserved 
fauna;  certainly  no  epoch  in  the  Great  Basin  can  surpass  it  in  general  in- 
terest, either  in  the  variety  of  its  organic  forms,  in  the  number  of  species 
determined,  or  in  the  commingling  of  species  found  elsewhere  in  widely 
separated  localities.  This  terrane  alone  has  yielded  more  species  than  the 
Cambrian  and  Silurian  periods  together,  and  surpasses  the  entire  Carbonif- 
erous, with  its  great  thickness  and  wide  areas,  by  more  than  one  hundred 
specific  forms.  From  Eureka  and  White  Pine  together  it  has  furnished 
over  two  hundred  species,  of  which  one-third  have  been  described  for  the 
first  time  in  the  report  of  Mr.  C.  D.  Walcott;1  while,  a  fact  of  great  interest 
as  regards  geographical  distribution,  one  hundred  and  nineteen  of  them  are 
specifically  identical  with  previously  described  forms  from  other  well  known 
Devonian  localities,  and  no  less  than  seventy-nine  of  them  have  been  identi- 
fied with  species  occurring  in  New  York.  The  Upper  Helderberg,  Hamil- 
ton, and  Chemung  are  all  well  represented  so  far  as  species  are  concerned, 
although  the  vertical  range  of  certain  species  by  no  means  agrees  with  the 
limits  assigned  to  them  in  New  York.  In  comparing  the  Nevada  limestone 
of  the  Great  Basin  with  the  Devonian  of  New  York  state,  Mr.  Walcott  says: 

'Paleontology  of  the  Eureka  District,  Monograph  VIII.     Washington,  1884. 


72  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

The  Upper  Helderberg  horizon  of  the  New  York  series  is  represented  by  thirty-eight 
species  common  to  it  and  the  lower  portion  of  the  Devonian  of  the  Eureka  district; 
the  Cheinung  group  of  the  same  by  sixteen  species;  of  the  Hamilton  species  of  New 
York  twenty-three  are  distributed  through  the  lower  portion  of  the  Eureka  Devonian 
limestone  and  eighteen  species  in  the  middle  and  upper  portions,  but  not  in  such  a 
manner  as  to  distinguish  a  middle  division  corresponding  to  the  Hamilton  formation 
of  New  York.  Of  strictly  Hamilton  species  in  New  York,  twenty-three  are  found,  of 
which  eleven  are  in  beds  a  little  below  the  summit,  and  twelve  just  above  the  base  of 
the  formation. 

Eleven  species  not  known  in  New  York  are  common  to  both  the 
Great  Basin  and  Iowa,  thus  emphasizing  the  faunal  relations  between  the 
corresponding  horizons  in  the  Cordillera,  the  Mississippi  Valley  and  the 
Appalachians. 

While  the  fauna  at  Eureka  is  rich  and  varied,  both  in  genera  and 
species,  remains  of  Devonian  fishes  appear  to  be  restricted  to  a  single 
ctenacaiithus-like  tooth.  Mr.  S.  F.  Emrnons,  while  engaged  on  the  Fortieth 
Parallel  Exploration,  brought  in  a  small  tooth  of  the  genus  Cladodm 
from  the  western  entrance  to  Emigrant  Canyon,  in  the  Tucubit  Moun- 
tains north  of  Humboldt  River.  These  two  single  specimens,  collected 
at  widely  separated  points,  are  all  that  is  known  of  Devonian  fishes  from 
Central  Nevada,  although  from  Northern  Arizona,  in  the  Kauab  Canyon, 
Mr.  C.  D.  Walcott1  obtained  abundant  evidence  of  the  presence  of  placo- 
ganoid  fishes  from  Devonian  beds,  which  were  represented  by  only  100 
feet  of  strata  as  against  8,000  feet  in  Nevada. 

Corals  occur  throughout  the  Nevada  limestone  and  certain  species 
present  a  wide  vertical  range.  Among  these  Stromatopora  are  known  from 
base  to  summit,  and  in  one  or  two  horizons  they  are  found  in  such  profusion 
as  to  characterize  the  strata  by  the  peculiar  weathering-out  of  the  imbedded 
silicified  corals.  In  the  siliceous  limestone  of  the  Upper  Devonian,  fragments 
of  Syringopora  associated  with  Stromatopora  are  occasionally  abundant  when 
all  other  species  are  wanting.  The  bedded  limestone  on  both  sides  of  the 
Yahoo  Canyon  offer  favorable  conditions  for  the  preservation  of  these 
forms.  Prior  to  the  survey  of  the  Eureka  District  the  Lamellibranchiates 
were  poorly  represented  from  the  Great  Basin.  To  a  meager  list  almost 

1  Am.  Jour.  Sci.  Sept.,  1880. 


DEVONIAN  FAUNA.  73 

wholly  collected  by  the  Geological  Exploration  of  the  Fortieth  Parallel, 
Eureka  has  now  furnished  no  less  than  twenty-three  genera 'and  thirty-five 
species. 

In  the  collections  from  Eureka,  occur  two  species,  first  described  by 
Mr.  F.  B.  Meek/  Orthis  macfarleni  and  EhynchoneUa  castanea,  from  the  Mac- 
kenzie River.  Both  of  these  important  species  were  brought  to  this  country 
by  the  late  Mr.  Robert  Kinnicut,  and  were  found  associated  together  on 
the  Lockhart  River,  a  tributary  of  the  Mackenzie,  in  latitude  67°  15'  north, 
longitude  126°  west,  while  the  Orthis  was  also  obtained  in  a  very  similar 
limestone  40  miles  below  Fort  Good  Hope,  on  the  Mackenzie.  According 
to  Mr.  A.  K.  Isbister,"  who  traveled  extensively  in  Northern  British  Amer- 
ica, along  the  base  of  the  Rocky  Mountains,  and  who  published  a  sketch 
map  of  its  geology,  the  Devonian  extends  through  the  valley  of  the  Macken- 
zie from  its  mouth  southward  for  15°  of  latitude,  nearly,  if  not  quite,  to  the 
headwaters  of  the  Saskatchewan  River.  It  certainly  is  of  considerable 
interest  to  find  these  two  species,  which  occur  together  in  the  Arctic 
regions,  associated  at  Eureka  in  the  upper  members  of  the  Lower  Devonian 
They  are  found  near  Woodpeckers  Peak,  about  3,000  feet  above  the  base 
of  the  limestone,  while  B.  castanea  was  also  obtained  from  the  upper  hori- 
zon at  Rescue  Hill. 

Within  the  area  covered  by  the  Nevada  limestone  collections  of  fossils 
were  made  more  or  less  complete  from  nearly  forty  localities.  For  the  pur- 
pose of  this  volume  it  seems  hardly  desirable  to  publish  the  lists  in  full, 
and  such  only  are  made  use  of  as  may  be  necesesary  to  elucidate  for 
geological  purposes  the  faunal  development  and  also  to  point  out  clearly 
upon  what  evidence  the  division  into  two  groups  is  based.  Of  the  6,000 
feet  included  within  the  epoch,  4,000  are  provisionally  assigned  to  the  lower 
and  2,000  to  the  upper  horizon.  About  two-thirds  of  the  species  belong  to 
the  lower  and  one-third  to  the  upper,  corresponding  roughly  to  the  relative 
thicknesses  of  the  two  horizons.  The  upper  portion  of  the  limestone, 
however,  represents  a  fauna  equally  varied,  although  not  so  complete,  as 
the  lower.  So  far  as  they  have  been  studied  the  upper  and  lower  horizons 
furnish  quite  characteristic  faunas,  with  only  seventeen  species  which  may 

'Trans.  Chi.  Acad.  Sci.,  vol.  i,  pt.  1,  1867-'69,  p.  88. 

3  Quarterly  Journal,  Geological  Society,  vol.  xi,  London,  1855,  p.  497. 


74  GEOLOGY  OF  THE  ETTEEKA  DISTEICT. 

be  considered  as  common  throughout  the  epoch.      The  following  list  com- 
prises the  species  common  to  both  upper  and  lower  horizons: 

Stromatopora,  ?  Productus  shumardianus,  var.  pyxidatus. 

Syringopora  perelegans.  Productus  subaculeatus. 
Streptorhynchus  chemungen-    Spirifera  pinonensis. 

sis  var.  pandora.  Spirifera  (M.)  maia. 

Orthis  tulliensis.  Atrypa  recticularis. 

Strophodonta  perplana.  Bhynchonella  castanea. 

Chonetes  deflecta.  Nyassa  parva. 

Productus  hallanus.  Paracyclas  occidentalis. 

Productus  shumardianus.  Styliola  flssurella. 

A  complete  systematic  list  of  all  the  genera  and  species  known  from 
the  Nevada  limestone  at  Eureka  and  White  Pine  tabulated  into  an  upper 
and  lower  group,  will  be  found  as  an  appendix  at  the  end  of  this  volume. 

At  Eureka,  above  the  light  gray,  crystalline  strata  carrying  the 
Halysites,  and  somewhere  near  the  base  of  the  Nevada  limestone,  the  beds 
begin  to  yield  Atrypa  reticularis,  Spirifera,  Stromatopora,  and  Edmondia, 
which  have  a  wide  vertical  range,  all  but  the  latter  extending  well  up 
nearly  to  the  top  of  the  limestone.  The  lowest  well  denned  fossiliferous 
belt  carrying  a  decided  Devonian  fauna  is  found  at  Lone  Mountain  not 
far  above  the  Silurian  line.  The  fauna  is  uncommonly  rich  in  species,  no 
one  locality  having  furnished  quite  as  many  forms.  They  occur  in  shaly 
strata  in  belt  No.  2  of  the  Lone  Mountain  section.  No  less  than  fifty-two 
species  were  obtained  from  this  horizon.  The  list  of  fossils  is  as  follows: 

Liiigula  Ifena.  Strophodonta  perplana. 

Liugula  lonensis.  Strophodonta  puuctulifera. 

Discina,  sp.  ?.  Chonetes  filistriata. 

Pholidops  bellula.  Chonetes  hemispherica. 

Pholidops  quadrangularis.  Chonetes  macrostriata. 

Orthis  inipressa.  Productus  shumardianus. 

Skeuidium  devonicum.  Productus  subaculeatus. 
Streptorhyuchus  chemungensis,  var.        Productus  navicella. 

perversa.  Spirifera  piiioneusis. 

Strophomena  rhomboidalis.  Spirifera  raricosta. 

Strophodonta  arcuata.  Spirifera  varicosa. 

Strophodonta  calvini.  Nucleospira  concinna. 

Strophodonta  pattersoni.  Trematospira  infrequens. 


DEVONIAN    FAUNA.  75 

Atrypa  desquamata.  Paracyclas  occidentalis. 

Atrypa  reticularis.  Microdon  macrostriata. 

Meristella  nasuta.  Anadontopsis  amygdalffifonnis. 

Ehynchouella  tethys.  Sckizodus  orbicularis. 

Cryptonella  circula.  Platyceras  nodosum. 

Pentamerus  comis.  Loxonema  nobile. 

Pterinea  flabella.  Bellerophon  pelops. 

Mytilarca  dubia.  Tentaeulites  gracilistriatus. 

Plethomytillis  oviforme.  Orthoceras  (2  sp.t). 

Modiomorpha  altiforme.  Beyrichia  occidentalis. 

Modiomorpha  obtusa.  Phacops  rana. 

Goniophora  perangulata.  Dalmanites  rneeki. 

Megainbonia  occidualis.  Proetus  marginalia. 
Edmondia  pifionensis. 

About  500  feet  above  this  belt,  in  the  dark  gray  limestone,  occurs  a 
group  of  fossils,  mainly  silicified  corals,  as  follows: 

Paleomanott  roemeri.  Cyathophyllum  davidsoni.    • 

Stroinatopora.  Cyathopliyllum  rugosuin. 

Favosites  basaltica.  Diphyphyllum  simcoense. 

Favosites  hemispherica.  Cystiphyllum  americanum. 

Favosites,  n.  sp.  Zaphrentis,  sp. !. 

Syringopora  perelegans.  Atrypa  reticularis. 

Above  this  latter  grouping  only  a  few  fossils  were  found,  mainly  species 
like  Atrypa  reticularis  and  Styliola  fissurella,  which  occur  all  through  the 
epoch.  In  this  long  list  of  species  from  the  base  of  the  Devonian  at  Lone 
Mountain,  only  seven  forms  occur  which  are  known  in  the  Upper  Devonian, 
the  list  as  a  group  being  decidedly  Lower  Devonian  in  character.  Skenidiwn 
devonicum  is  the  only  species  of  this  genera  which  is  known  above  the 
Silurian,  while  Atrypa  desquamata,  here  associated  with  A.  reticularis,  occurs 
only  in  the  lower  beds.  The  Devonian  trilobites  in  this  list  occur  in 
nearly  all  the  other  fossil-bearing  beds  at  the  base  of  the  Nevada  limestone — 
namely,  Combs  Peak,  Atrypa  Peak,  Brush  Peak — but  are  not  found  in  the 
middle  or  upper  horizons.  It  will  be  noticed  that  the  list  includes  quite  a 
number  of  species  usually  regarded  as  characteristic  types  of  the  Upper 
Helderberg. 

At   Combs  Mountain,  Atrypa  Peak,  Brush  Peak,  Modoc  Peak,  and 
several  other  localities  occur  fossiliferous  calcareous  shale  bands  well  defined 


76  GEOLOGY  OF  THE  EUKEKA  DISTRICT. 

lithologically,  which  present  much  the  same  aspect  at  each  place,  with  a 
similar  Lower  Devonian  fauna,  many  of  the  forms  being  specifically 
identical.  The  evidence  goes  to  show  that  they  belong  essentially  to  the 
same  horizon,  although  the  estimated  vertical  distance  of  the  beds  above 
the  Eureka  quartzite  varies  considerably  in  the  different  localities.  This 
difference  is  undoubtedly  due  in  part  to  the  varying  thickness  of  the  under- 
lying Lone  Mountain  beds  resting  on  the  quartzite  and  partly  to  the  more 
rapid  changes  in  some  places  than  in  others  in  the  nature  of  the  sedi- 
mentation. In  certain  localities,  under  favorable  conditions,  the  cal- 
careous shale  seems  to  have  been  deposited  earlier  than  elsewhere.  In 
other  words,  the  shale  belts  are  not  absolutely  synchronous ;  in  some  places 
they  are  known  to  be  wanting.  They  may  be  taken  as  representing  char- 
acteristic horizons  in  the  Lower  Devonian  without  at  the  same  time  occu- 
pying a  sufficiently  definite  position  to  be  made  a  datum  point  in  deter- 
mining the  thickness  of  the  strata  between  the  shale  belt  and  the  basal 
member  of  the  epoch. 

The  following  list  includes  all  species  obtained  from  the  calcareous 
shale  belts  of  Brush  Peak,  Atrypa  Peak,  and  Combs  Mountain.  The 
numerals  affixed  opposite  the  name  of  each  species  indicate  from  which  of 
the  three  localities  they  have  been  obtained.  In  this  way  it  will  be  seen 
at  a  glance  which  forms  are  common  to  more  than  one  of  these  typical 
localities. 

—  3  Strom  atopora.  -  2  -  Strophodonta  demissa. 

-  2  -  Favosites  basaltica.  -23  Strophodonta  inequiradiata. 
1-3  Favosites  n.  sp.  -  2  3  Strophodonta  perplaua. 

—  3  Pachyphyllum  woodmani.         -  2  -  Strophodonta  punctilifera. 

-  2  3  Zaphrentis.  1  2  -  Chonetes  deflecta. 

-  2  -  Lingula  whitei.  -  2  3  Chonetes  filistriata. 

1  2  -  Orthis  impressa.  1  —  Chonetes  granulifera. 

123  Streptorhynchus  chemungen-  1  —  Chonetes  hemispherica. 

sis.  1  2  -  Chonetes  niacrostriata. 

-  2  3  Streptorhynchus  chemungen-  -  2  -  Productus  navicellus. 

sis,  var.  pandora.  -  2  -  Productus  subaculeatus. 

-  2  -  Streptorhynchus  chemuugen —  2  -  Productus  truncata. 

sis,  var.  perversa.  123  Spirifera  pinouensis. 

1-3  Strophodonta  calvini.  -  2  -  Spirifera  undifera, 


DEVONIAN  FAUNA.  77 

-  3  Spirifera  sp.?.  l  _  3  Platyceras  conradi. 

-  2  -  Atrypa  desquamata.  -  2  -  Platyceras  dentalium. 
123  Atrypa  reticularis.  I  -  -  Platyceras  thetiforme. 
1  2  -  Rhynchonella  horsfordi.  -  3  Platyceras  thetis. 

-  -  3  Rhynchonella  occidens.  1  -  -  Platyceras  undulatum. 

-  -  3  Rhynchonella  tethys.  1-3  Platyostoma  lineata. 

-  3  Pentamerus  comis.  -  2  -  Ecculiomphalus  devonicus. 

-  2  -  Leipteria  rafinesqui.  -  2  3  Euomphalus  eurekensis. 

-  2  -  Limoptera  sarmentica.  -  3  Calonema  occidentalis. 

-  2  -  Mytilarca  sp.!.  -  2  -  Cyclonema  (like  C.  midtilera). 

-  2  -  Modiomorpha  oblouga.  -  2  -  Loxonema  approximatum. 
1  —  Modiomorpha  obtusa.  -  2  3  Loxonema  uobile. 

-  2  -  Goniophora  perangulata.  -  3  Loxonema  subattenuata. 

-  2  3  Edmondia  pifionensis.  -  2  -  Bellerophon  neleus. 

-  3  Sanguinolites  combensis.  123  Bellerophon  perplexa. 

1  -  -  Sanguinolites  gracilis.  -  2  -  Scoliostoma  americana. 

-  2  -  Sanguinolites  sauduskyensis.    1  -  -  Tentaculites  attenuatus. 
—  3  Conocardium  iievadensis.  -  -  3  Tentaculites  scalariformis. 

-  2  -  Posidomya  devonica.  -  2  -  Hyolithes  sp.  ?. 

-  2  -  Posidomya  laevis.  -  2  -  Orthoceras  sp.  ?. 

-  2  -  Microdon  macrostriata.  -  3  Goniatites  desideratus. 

-  2  -  Schizodus  orbicularis.  123  Phacops  rana. 

-  2  -  Cypricardinia  iudenta.  123  Dalmanites  meeki. 

-  2  3  Platyceras  carinatum.  -  3  Proetns  marginalis. 

[No.  1,  from  the  south  slope  of  Brush  Peak.     No.  2,  from  the  shale  belt  of  Atrypa  Peak.     No.  3,  from 

the  west  Bpur  of  Combs  Mountain.] 

The  shale  belt  of  Brush  Peak  promises  to  the  collector  a  most  varied 
fauna  of  Lower  Devonian  species.  It  measures  about  150  feet  in  thickness 
and  may  be  traced  along  the  west  side  of  both  Brush  and  Modoc  peaks; 
thence  still  farther  northward,  where  its  connection  is  clearly  made  out 
with  shale  belt  No.  3,  of  the  Devonian  section,  south  of  Signal  Peak. 
On  the  southeast  slope  of  Atrypa  Peak  the  shale  belt  crosses  the  spur 
striking  N.  30°  E.,  dipping  40°  W.  The  beds  are  of  a  light  bluish  gray 
color  about  150  feet  in  thickness.  The  horizon  corresponds  to  the  fos- 
siliferous  shale  belt  in  the  section  east  of  Lamotireux  Canyon  (p.  67). 

Combs  Mountain  presents  upon  its  south  side  a  fine  display  of  massive 
limestone  beds  dipping  northward  into  the  mountain.  There  is  exposed 
here  between  the  base  of  the  mountain  and  the  summit  of  the  ridge 


78  GEOLOGY  OF  THE  EUEEKA  DISTRICT. 

nearly  5,000  feet  of  strata.  No  line  of  demarcation  can  be  drawn 
here  between  the  Lone  Mountain  and  Nevada  epochs.  Fossils  were 
rarely  met  with  except  in  well  denned  strata,  separated  by  long  ver- 
tical intervals.  The  Trenton  horizon,  which  is  well  represented,  is  esti- 
mated at  300  feet  in  thickness,  resting  immediately  upon  the  Eureka  beds. 
From  the  top  of  the  Trenton  the  section  across  the  beds  is  strikingly  similar 
to  those  observed  at  Atrypa  and  Brush  peaks.  Careful  estimates  place  the 
fossiliferous  shale  at  1,700  feet  above  the  Trenton  or  2,000  feet  above  the 
Eureka  quartzite.  This  is  the  same  vertical  distance  above  the  quartzite 
assigned  to  the  shale  belt  at  Atrypa  Peak,  although  at  the  latter  locality 
the  Trenton  limestone  is  not  recognized  either  by  its  physical  features  or 
its  organic  forms.  From  the  shale  belt  to  the  top  of  the  ridge  the  only 
species  secured  were  corals  having  a  wide  vertical  range  or  else  fragments 
too  imperfect  for  specific  description.  A  comparison  of  the  species  obtained 
in  the  three  shale  belts,  taken  together  with  the  stratigraphy  of  the  beds, 
proves  without  much  doubt  the  equivalency  of  the  Combs  Mountain  shale 
with  those  at  Atrypa  and  Brush  peaks. 

In  the  County  Peak  body  of  limestone  the  lowest  organic  remains 
obtained  occur  midway  in  the  siliceous  limestone  beds  of  No.  6,  of  the 
County  Peak  section  (p.  68).  Here  the  gray  and  blue  limestone  of  No. 
8  is  assigned  to  the  base  of  the  Devonian,  which  places  the  fossil-bearing 
bed  about  1,000  feet  above  the  Silurian.  The  species  recognized  are 
Edmondia pinonensis,  Atrypa  reticularis,  Spirifera  sp.  ?.  and  Cladopora  sp.  I. 

Passing  upward  for  2,000  feet  above  this  last  bed,  or  3,000  feet  above 
the  base,  and  in  about  the  middle  of  the  great  limestone  belt  (No.  2),  there 
occurs  in  a  thinly  bedded  bluish  gray  limestone  an  interesting  grouping  of 
species  characteristic  of  the  middle  Devonian,  or  rather  a  mingling  of 
species  from  both  upper  and  lower  horizons.  The  bed,  owing  to  its 
marked  lithological  features,  may  be  traced  by  the  eye  for  long  distances 
along  the  slope  of  the  mountains.  At  Woodpeckers  Peak,  where  the  col 
lection  was  made,  the  fauna  is  by  no  means  as  large  or  as  varied  as  that 
found  in  the  lower  shale  belt.  While  many  species  are  identical  with  those 
found  at  the  lower  horizon,  and  present  a  decided  Lower  Devonian  aspect, 
the  greater  part  of  them  are  common  to  both  Upper  and  Lower  beds.  It  is 


UPPER  DEVONIAN  FAUNA.  79 

at  this  locality  that  the  two  Mackenzie  River  species  are  seen  associated 
together  in  the  same  matrix.  The  following  is  the  list  of  species  collected  at 
Woodpeckers  Peak: 

Orthis  macfarleni.  Productus  truncatus. 

Streptorhynchus  chemungensis,  var.  pandora.  Spirifera  (M.)  maia. 

Streptorhynchus  chemungeiisis,  var.  perversa.  Atrypa  reticulans. 

Ehynchonella  castanea.  Nyassa  parva. 

Strophomena  rhomboidalis.  Edmondia  piuonensis. 

Chonetes  deflecta.  Paracyclas  occidentalis. 

Productus  hallanus.  Metoptoina  devonica. 
Productus  subaculeatus. 

On  the  south  slope  of  Sentinel  Peak,  southeast  of  the  last  locality,  at 
about  the  same  horizon  as  the  grouping  of  fossils,  a  small  collection 
was  obtained,  all  but  two  of  them  being  identical  with  those  observed  at 
Woodpeckers  Peak,  and  all  of  them,  without  exception,  forms  recognized 
from  the  Upper,  as  well  as  the  Lower,  horizon.  The  two  species  not  known 
at  Woodpeckers  Peak  are  Styliola  fissurella  and  Lingula  ligea,  var.  nevadensis, 
the  former  common  tliroughout  the  Nevada  limestone,  and  the  latter  a 
Hamilton  species  of  New  York  state,  collected  also  from  Rescue  Hill,  of 
the  Upper  Devonian. 

Another  1,000  feet  of  limestone  reaches  the  dark  blue  massive  beds  in  the 
upper  part  of  No.  1  of  the  County  Peak  section.  If  the  somewhat  arbitrary 
line,  provisionally  drawn  between  the  Upper  and  Lower  Nevada  limestones, 
is  correctly  placed  about  4,000  feet  above  the  base  of  the  Devonian,  these 
beds  would  lie  at  the  base  of  the  upper  series.  In  all  probability  they  belong 
to  the  Upper  Nevada  limestone,  although  there  is  nothing  sufficiently  dis- 
tinct in  the  meager  fauna  obtained  to  determine  the  question  definitely. 
The  only  species  observed  which  is  at  all  restricted  in  its  range  is  Spirifera 
engelmanni,  a  form  common  to  the  highest  members  of  the  epoch,  but  no- 
where as  yet  found  lower  down  than  these  intermediate  strata.  Somewhat 
higher  beds  give  much  the  same  grouping  of  fossils,  and  in  several  localities 
Spirifera  engelmanni  has  been  recognized.  The  highest  horizon  in  this 
great  mass  of  limestone  from  which  fossils  have  been  obtained  is  in  a  well 
stratified  blue  bed  near  the  mouth  of  Packer  Basin,  where  the  fauna  has  a 
decidedly  Upper  Devonian  aspect.  Among  the  species  collected  here  are 


80  GEOLOGY  OF  THE  EUEEKA  DISTRICT. 

Spirifera  engelmanni  and  the  two  Chemung  forms,  Rliynchonella  duplicate  and 
E.  sinuata,  both  fcmnd  at  several  localities  in  the  Upper  Nevada  limestone. 
Rescue  Hill,  on  the  east  side  of  Rescue  Canyon,  is  a  faulted  block  of 
Devonian  limestone.  Along  the  abrupt  east  slope  of  the  hill  the  north  and 
south  Rescue  Canyon  fault  cuts  off  the  limestone  from  that  found  on  the 
opposite  side  of  the  canyon,  while  an  east  and  west  fault,  approximately 
coinciding  with  the  course  of  Silverado  Canyon,  intersects  the  Rescue  Can- 
yon fault,  and  separates  the  hill  from  the  limestone  body  to  the  north. 
The  beds  forming  the  summit  of  Rescue  Hill  belong  to  strata  somewhat 
higher  in  the  series  than  those  found  on  the  summit  of  Sentinel  Peak  and 
Island  Mountain,  but  the  lower  limestones  of  the  three  localities  may  be 
easily  correlated.  In  a  light  bluish  gray  limestone  just  below  the  top  of 
Rescue  Hill  the  following  grouping  of  fossils  occurs : 

Lingula  ligea,  var.  iievadensis.     Mytilarca  chemungensis. 

Productus  hallanus.  Leptodesma  transversa. 

Productus  shumardianus.  Nucula  rescueusis. 

Productus  stigmatus.  Nucula  (like  N.  niotica,  Hall). 

Productus  subaculeatus.  Grammysia  minor. 

Spirifera  (M.)  maia.  Sanguinolites  ventricosus. 

Atrypa  reticularis.  Paracyclas  occidentalis. 

Rhynchonella  castanea.  Platyceras  carinatus. 

Rhynchoiiella  duplicata.  Bellerophon  pelops. 

Ehynchouella  (L.)  laura.  Naticopsis,  sp.f  (like  If.  aequistriata,  Meek). 

Rhynchonella  iievadensis.  Tentaculites  gracilistriatus. 

Rhynchonella  siuuatus.  Styliola  fissurella. 

Cryptonella  piuonensis.  Proetus  haldermanni. 

The  Rescue  Canyon  fault,  as  already  described,  is  a  profound  displace- 
ment. After  crossing  Silverado  Canyon  at  the  head  of  Rescue  Canyon,  it 
extends  northward  until  concealed  beneath  the  great  basalt  flow.  By 
reference  to  the  map  (atlas  sheets  vm  and  x)  the  course  of  the  fault  will 
be  seen  along  the  base  of  Sugar  Loaf  and  Sentinel  Peak.  In  the  faulted 
block  to  the  eastward  there  occurs  a  wedge-shaped  mass  of  Devonian 
limestone  lying  north  of  Silverado  Canyon  and  east  of  Island  Mountain 
and  Sugar  Loaf.  It  conformably  underlies  the  great  body  of  White  Pine 
shale  and  admirably  shows  the  relation  between  the  Nevada  limestone 
and  the  overlying  shale.  These  beds  directly  underlying  the  shale  are  of 


WHITE  PINE  SHALE  FAUNA.  81 

course  the  uppermost  members  of  the  Nevada  limestone.  The  following 
section  gives  the  sequence  of  beds  from  the  Quaternary  plain  westward 
across  the  White  Pine  shale  and  the  underlying  limestone  until  the  beds 

are  cut  off  by  the  fault. 

i ...  • 

1.  Shaly  sandstone  followed  by  50  foot  of  dark  argillaceous  shale  and  a  great  thickneas  of 

arenaceous  shale  and  thinly  bedded  sandstone  ;  occasional  lied*  of  fine  siliceous   con- 
glomerate ;  constant  changes  from  shale  to  sandstone 1,000 

2.  Black  argillaceous  shale  passing  into  arenaceous  shale  and  shaly  sandstone  becoming  dis- 

tinctly bedded  and  passing  up  into  a  fine  siliceous  conglomerate.     Throughout  the  series 

are  occasional  thin  belts  of  argillaceous  shale 400 

3.  Gray  criuoidal  limestone  in  layers  of  varying  thickness  and  more  or  less  sandy  ;  carries 

Chonetea 50 

4.  Dark  bluish  black  argillaceous  and  calcareous  shale  weathering  yellow  on  the  surface  ; 

fossiliferons 300 

5.  Blue  limestone  with  alternating  thin  massive  layers  ;  fossiliferous 250 

6.  Siliceous  limestone  passing  into  gray  limestone  with   irregular  seams  and  nodules  of 

calcite 150 

The  lower  gray  limestone  carries  no  fossils. 

In  the  massive  blue  limestone  (No.  5)  occur  the  following  Upper 
Devonian  species: 

Productus  shumardianus.  Ehynchonella  duplicate. 

Spirifera  eiigelmanni.  Leperditia  rotundatus. 

Atrypa  reticularis.  Styliola  flssurella. 

In  the  overlying  300  feet  of  clay  shales  (No.  4)  the  more  calcareous 
portions  carry  Spirifera  engelmannl  and  Productus  shumardianus,  while  in 
the  more  argillaceous  strata  are  numerous  imperfect  plant  remains. 

The  gray  limestone  (No.  3)  overlying  the  black  shale  is  characterized 
by  typical  Devonian  forms:  Chonetes  mucronata,  Spirifera  ctiijclnuinni  and 
Beyrichia  occidentalis.  Above  this  latter  limestone  in  the  clayey  and  sandy 
strata  (Nos.  1  and  2)  no  invertebrate  forms  have  as  yet  been  obtained,  but 
numerous  fragments  of  plant  remains,  some  of  which  would  doubtless 
admit  of  generic  determination,  are  abundant.  A  careful  search  for  a 
Devonian  flora  would  yield  important  results.  The  evidence  of  the 
Devonian  age  of  the  upper  1,400  feet  of  shales  and  sands  is  apparent, 
from  the  identity  of  the  plants  with  those  obtained  from  the  black  shale 
below  the  gray  limestone  as  well  as  from  the  character  of  the  sediments. 
Another  locality  where  the  Nevada  limestone  and  White  Pine  shale  are 
MON  xx (i 


82  GEOLOGY  OF  THE  EUBEKA  DISTRICT. 

structurally  well  shown  with  a  typical  fauna  in  both  horizons  is  found  at 
Newark  Mountain.  The  mountain  presents  a  bold  impressive  mass  of 
bluish  gray  limestone  with  the  physical  features  of  the  Upper  Devonian 
strata.  The  section  here  is  as  follows: 

Feet. 

1.  Black  argillaceous  shale  more  or  less  arenaceous  and  similar  to  the  lower  black  shale 1,000 

2.  Compact  fine  grained  sandstone  with  minute  dark  siliceous  pebbles   scattered  through 

the  beds 100 

3.  Black  argillaceous  shale  with  fine  intercalated  beds  of  arenaceous  shale.     These  shales 

crumble. oil  exposure  to  atmospheric  influence 500 

4.  Reddish  gray  shaly  calcareous  beds 100 

5.  Dark  gray  heavily  bedded  siliceous  limestone  passing  into  bluish  gray  limestone  in  places 

finely  banded 3,500 

Several  hundred  feet  below  the  top  of  the  Nevada  limestone  and  cal- 
careous shale  the  limestone  yielded  a  small  group  of  fossils,  some  of  them 
common  to  both  the  upper  and  lower  horizons,  but  none  of  them  character- 
istic of  the  Lower  Devonian. 

Stromatopora.  Spirifera  pifiouensis. 

Strophodonta  perplana.  Arrypa  reticularis. 

Producing  shamardianns.  Pterinea  newarkeusis. 

Spirifera  disjuncta.  Platyschisma  maccoyi. 

Immediately  below  the  black  shales,  near  the  eastern  end  of  Newark 
Mountain,  the  folio  wing  species  occur: 

Ortliis  tulliensis.  Atrypa  reticularis. 

Spirifera  disjuncta.  Nyassa  parva. 

Spirifera  engelmanni.  Straparollus  newarkensis. 

Athyris  angelica.  Beyrichia  occidentalis. 

Reddish  gray  calcareous  shales  pass  rapidly  into  the  argillaceous 
beds.  Invertebrate  remains  wherever  found  in  the  black  shale  are  imper- 
fectly preserved  so  that  specific  determinations  are  in  most  instances  out  of 
the  question.  From  the  lower  beds  were  obtained  Auiculopecten  and  a 
species  of  Goniatites,  while  the  upper  and  rather  more  sandy  beds  have 
furnished  a  more  varied  material  in  which,  according  to  Mr.  C.  D.  Walc<>Tt, 
the  facies  is  Devonian  with  a  foreshadowing  of  the  Carboniferous  period. 
Among  the  genera  found  here  are  Fenestettu)  Chonetes,  MotKomorpha,  sp.  ?. 
Cypricardinia,  sp.f,  Palaeoneilo,  sp.f,  Cardiomorpha,  sp.?,  Conocardium,  sp.  ?, 
and  Goniatites.  In  only  two  cases  were  specific  determinations  possible: 


FAUNA.   OF   YAHOO  CANTOM.  83 

Productus  hirsutiforme  and  Coholm  Icevix.  Plant  remains  occur  here  sim- 
ilar to  those  found  east  of  Sugar  Loaf,  but  still  less  perfectly  preserved. 
The  identification  of  the  flora  from  the  former  locality  places  the  age  of 
these  beds  without  doubt  at  the  top  of  the  Devonian,  in  accordance  with 
their  stratigraphical  position.  The  corresponding  horizon  at  White  Pine 
Mountain  presents  still  stronger  evidence  of  the  Devonian  age  of  the 
shale;  but  here,  as  well  as  at  Eureka,  little  has  been  accomplished  by 
investigating  this  ancient  flora. 

Passing  to  the  Pinon  Range  and  the  Mahogany  Hills  in  the  northwest 
corner  of  the  district,  the  Upper  Devonian  limestone  is  well  exposed  in 
massive  beds  lying  beneath  the  Diamond  Peak  quartzite.  It  is  easily  deter- 
mined by  its  lithological  habit  and  fauna,  as  well  as  by  its  geological 
position  beneath  the  Carboniferous  quartzite.  Fossils  are  known  in  a  num- 
ber of  places,  but  the  localities  which  have  furnished  the  largest  and  most 
varied  fauna  and  offer  the  most  promising  return  are  found  on  the  east  side 
of  Yahoo  Canyon  and  north  side  of  The  Gate.  Near  the  entrance  to 
Yahoo  Canyon  the  beds  have  yielded  a  rich  fauna  characterized  by  silici- 
fied  corals.  The  grouping  here  is  as  follows: 

Strornatopora.  Pachyphyllum  woodmuui. 

Alveolites  rockfordeiisis.  Spirifera  glabra,  var.  nevadensis. 

Cladopora  pulchra.  Spirifera  disjnncta. 

Syringopora  hisingeri.  Atrypa  recticulans. 

Syringopora  perelegaus.  Bhynchonella  c.astanea. 

Cyathophyllura  corniculuni.f  Styliola  fissurella. 

On  the  north  side  of  The  Gate,  at  a  little  higher  horizon  and  directly 
beneath  the  quartzite,  there  is  exposed  a  fine  section,  500  feet  in  thickno-. 
of  massive  blue  limestone,  passing  into  shaly  beds,  in  places  almost  fissile. 
Fossils  characteristic  of  the  Upper  Devonian  are  abundant  throughout  the 
beds.  The  limestone  yielded  the  following  species: 

Stromatopora.  Orthis  tulliensis. 

Syringopora  hisiugeri.  Productus  lachrymosa,  var.  liina. 

Syringopora  perelegans.  Productus  schninardiauus. 

Cyathophyllum  corniculum.  Productus  speciosus. 

Disciua  iniuuta.  Productus  stiginatus. 

Orthis  impressa,  Productus  subaculeatus. 


84  (iEOLOGY  OF  THE  EUKEKA  DISTRICT. 

Spirifera  disjuncta.  Sanguinolites  rigidus. 

Spirifera  engelmanni.  Paracyclas  occidentalis. 

Athyris  angelica.  Euomphalus  (P.)  laxus. 

Atrypa  reticularis.  Euomphalus,  sp.? 

Ehynchonella  pugnus.  Platyschisma  1  ambigua. 

Bhynchonella  (L.)  laura.  Naticopsis,  sp.? 

Ehynchonella  (L.)  nevadensis.  Styliola  fissurella. 

Ehynchonella  (L.)  sinuata.  Cytoceras  nevadensis. 

Grammysia  minor.  Orthoceras,  sp. ! 

It  is  immediately  overlying  the  limestone  holding  this  fauna  that  the 
argillaceous,  cherty  beds  occur  which  carry  poorly  preserved  fragments  of 
plant  remains  and  the  single  species,  Discina  minuta.  They  probably 
represent  the  great  development  of  the  White  Pine  shale  found  upon  the 
east  slope  of  Newark  Mountain,  but  they  are  not  represented  on  the  map, 
as  they  are  recognized  only  in  a  few  localities  lying  between  the  Nevada 
limestone  and  Diamond  Peak  quartzite. 

CARBONIFEROUS    ROCKS. 

Although  rocks  of  this  period  cover  large  areas  and  make  up  the 
greater  part  of  many  mountain  ridges  in  the  Great  Basin,  few  localities 
offer  better  exposures  of  all  the  epochs  into  which  they  have  been  divided 
than  that  portion  of  the  Diamond  Range  which  lies  within  the  limits  of  the 
Eureka  survey.  To  the  northeast  and  east  of  Eureka,  Carboniferous  rocks, 
more  especially  the  limestones,  present  a  greater  thickness  of  strata  than  is 
shown  here,  but  inmost  cases  the  single,  narrow  ridges  fail  to  expose  in  anv 
continuous  section  the  entire  series  of  rocks  from  base  to  summit,  At 
Eureka  the  Carboniferous  rocks  have  been  estimated  to  measure  9,300  feet 
in  thickness,  which,  however,  does  not  represent  the  full  development  of 
the  Carboniferous  period,  the  Upper  Coal-measures,  the  top  of  the  Paleozoic 
system  having  suffered  a  very  considerable  amount  of  erosion.  This  upper 
limestone  is  by  no  means  as  thick  as  that  found  elsewhere. 

The  Carboniferous  rocks  have  been  subdivided  into  four  epochs:  First, 
Diamond  Peak  quartzite;  second,  Lower  Coal-measure  limestone;  third, 
Weber  conglomerate;  fourth,  Upper  Coal-measure  limestone. 


DIAMOND  PEAK  QUAETZITE.  85 

Diamond  Peak  Quartzite.-This  epoch,  the  base  of  the  series,  takes  its  name 
from  Diamond  Peak,  where  it  is  exposed  on  both  flanks  of  the  peak,  dip- 
ping into  the  range  with  a  synclinal  structure.  On  the  west  side  of  the 
peak,  where  it  attains  its  greatest  exposure,  it  measures  about  3,000  feet  in 
thickness.  Beds  of  this  epoch  are  found  only  at  Diamond  Peak  and  on 
the  opposite  side  of  the  valley  in  the  region  of  The  Gate.  At  the  base  of 
the  horizon  fine  conglomerates  firmly  cemented  together  lie  next  the  argil- 
laceous shale  of  the  White  Pine  epoch,  but  quickly  give  place  to  a  more 
massive,  usually  vitreous,  quartzite  with  a  characteristic  grayish  brown  color 
and  breaking  irregularly  with  a  flinty  fracture.  Intercalated  black  cherty 
bands,  carrying  a  more  or  less  ferruginous  matter,  occur  near  the  middle 
portion  of  the  horizon.  Near  the  summit  the  beds  pass  into  thinly 
laminated  green,  brown  and  chocolate-colored  schists  and  clay  shales.  The 
Carboniferous  age  of  the  epoch  is  determined  by  a  narrow  belt  of  blue 
limestone,  which  occurs  iuterstratified  in  the  quartzite  about  200  feet  above 
its  base,  in  which  the  widespread  species  Productus  semireticulatus  occurs 
associated  with  an  undetermined  species  of  Athyris.  As  the  fauna  at  the 
top  of  the  black  shales  foreshadows  the  coming  in  of  the  Carboniferous,  the 
presence  of  this  characteristic  Productus,  with  only  a  Carboniferous  fauna 
higher  up  in  the  series,  determines  without  question  the  geological  position 
of  the  quartzite  between  the  black  shale  and  Coal-measure  limestone. 

Lower  Coal-measure  Limestone.— Beds  of  this  epoch  are  found  in  a  great 
number  of  ranges  in  Utah  and  Nevada,  stretching  all  the  way  from  the 
Wasatch  to  Battle  Mountain,  and  the  horizon  has  probably  been  better 
studied  than  any  other  in  the  Great  Basin.  The  beds  cover  large  areas  at 
Eureka  and  offer  better  exposures  than  any  other  division  of  the  Carbon- 
iferous. In  the  Diamond  Range  they  overlie  conformably  the  Diamond 
Peak  quartzite,  the  transition  beds  passing  rapidly  from  siliceous  to  cal- 
careous sediments.  In  their  lithological  character  and  physical  habit  they 
do  not  differ  essentially  from  the  same  beds  elsewhere,  except,  perhaj», 
at  their  base,  where  they  carry  intercalated  beds  of  chert,  argillite,  and 
gritty,  pebbly  limestone,  with  evidences  of  shallow  water  deposition.  They 
pass  rapidly,  however,  into  purer  gray  and  blue  limestone,  for  the  most 
part  heavily  bedded  and  distinctly  stratified  at  varying  intervals.  In 


86  GEOLOGY  OF  THE  EUKEKA  DISTRICT. 

broad  masses  they  resemble  the  Upper  Nevada  limestone,  but  are  rather 
lighter  in  color  in  distinction  from  the  dark  blue  and  black  of  the  latter 
horizon.  No  true  dolomite  beds  of  any  considerable  thickness  have  been 
recognized,  9 '21  per  cent  being  the  largest  amount  of  magnesium  carbon- 
ate obtained  in  any  of  the  rocks  subjected  to  chemical  analysis.  Across 
their  broadest  development  they  measure  about  3,800  feet  in  thickness, 
which  is  much  less  than  has  usually  been  assigned  to  this  horizon  in  other 
mountain  uplifts,  more  especially  those  lying  eastward. 

As  the  term  Lower  Coal-measure  has  been  employed  by  most  geolo- 
gists to  designate  this  epoch  throughout  the  Great  Basin,  it  has  been 
thought  best  to  retain  the  name  provisionally,  although  not  exactly  appli- 
cable, as  the  epoch  includes  such  a  commingling  of  species  from  both  the 
Upper  and  Lower  Coal-measures  that  a  separation  of  the  beds  seems  quite 
impossible.  Moreover,  those  distinctions  which  hold  good  in  the  Missis- 
sippi Valley  are  by  no  means  always  applicable  to  the  Cordillera.  In  the 
present  state  of  our  knowledge  of  the  Carboniferous  limestone,  it  is  impos- 
sible to  establish  subdivisions  in  either  of  the  Coal-measure  epochs,  based 
upon  faunal  differences,  owing  to  the  fact  that  so  many  species  extend 
through  a  wide  vertical  range,  and  so  few  characteristic  species  occur  within 
restricted  limits. 

Lower  Coal-measure  Fauna.— As  the  limestones  are  in  general  favorable  to 
the  preservation  of  organic  remains,  fossil-bearing  strata  are  found  through- 
out the  beds,  and  geologists  are  not  so  dependent  upon  definite  horizons 
as  among  Lower  Paleozoic  rocks.  About  100  species  have  been  collected 
from  this  epoch,  but  most  of  those  obtained  from  the  upper  and  middle 
portions  have  already  been  recognized  as  occurring  elsewhere  in  the 
Lower  Coal-measures  of  the  Great  Basin.  In  comparison  with  the  new 
species  obtained  from  the  Cambrian,  Silurian  and  Devonian,  the  Carbon- 
iferous of  Eureka  offer  singularly  few  forms  new  to  science,  but  this,  of 
course,  may  be  accounted  for  by  the  thorough  researches  which  have  been 
made  in  this  period  elsewhere.  At  the  base  of  the  limestone  the  life  is 
more  varied  and  presents  certain  facts  that  are  of  both  geological  and 
biological  interest. 

Three  salient  features  in  the  life  of  the  Lower  Coal-measures  at  Eureka 


CARBONIFEROUS  FRESH-WATER  LIFE.  87 

call  for  special  mention,  and  each  is  worthy  of  still  further  investigation: 
First,  the  occurrence  near  the  base  of  the  limestone  of  a  fresh- water  fauna; 
second,  the  varied  development  of  the  Lamettibranchiates,  a  class  which  has 
heretofore  been  but  sparingly  represented  in  the  collection  of  Carboniferous 
fossils  from  the  Cordillera ;  third,  the  mingling  near  the  base  of  the  horizon 
of  Devonian,  Lower  Carboniferous  and  Coal-measure  species  in  gray  lime- 
stone directly  overlying  beds  characterized  by  a  purely  Coal-measure  fauna. 

Fresh-water  Life.— The  lowest  strata  from  which  we  have  any  record  of 
organic  life  from  this  epoch  are  found  at  the  extreme  northeast  corner  of  New 
York  Mountain,  and  also  near  the  railway  cut  immediately  south  of  the 
Richmond  furnaces.  Both  localities  lie  just  east  of  the  Hoosac  fault,  which 
brings  up  Carboniferous  beds  against  the  Silurian.  But  for  the  alluvial 
deposits,  which  occupy  the  valley,  the  beds  of  the  two  localities  would 
probably  be  found  to  be  continuous;  the  rocks  in  botli  are  similar.  There 
occur  here  100  feet  or  more  of  fine  clays  and  grits,  interstratified  with 
arenaceous  and  argillaceous  limestones  passing  up  into  pure  limestone, 
showing  abrupt  changes  and  rapid  deposition.  In  these  transition  beds 
were  found  abundant  evidence  of  a  varied  fresh-water  life,  it  being  possi- 
ble to  determine  several  distinct  species.  The  shells  indicate  a  shallow 
water  fauna,  as  is  also  clearly  established  by  the  mode  of  deposition  of  the 
sediments.  Mingled  with  these  shells  are  a  few  fragmentary  bits  of  twigs 
and  stems  of  plant  life,  for  the  most  part  referable  to  a  coniferous  growth, 
and  showing  signs  of  having  been  washed  down  from  a  land  surface  that 
could  not  have  been  very  far  away.  Mr.  Walcott  has  briefly  described 
three  species:  one  belonging  to  the  genus  Physa,  named  by  him  P.  priscu; 
another  form  is  a  pulmonate  shell,  allied  to  the  genus  Auricula,  and  to 
which  he  has  given  the  name  Zaptychius  carboiiaria;  a  third  shell  is  related 
to,  if  not  identical  with,  Ampullaria,  and  is  provisionally  named  after  the 
Director  of  the  Geological  Survey,  A.  powetti.  The  discovery  of  fresh  or 
brackish  water  shells  so  low  down  in  the  Paleozoic  and  so  remote  from  any 
known  locality  of  similar  beds  renders  their  mode  of  occurrence  one  of 
peculiar  interest. 

Lameiubranchiate  Fauna.— From  the  horizon  of  the  Lower  Coal-measures 
there  have  been  collected  over  forty  species  of  Lamellibranchiate  shells,  a 


88  GEOLOGY  OP  THE  EUREKA  DISTRICT. 

class  which  heretofore  has  been  but  sparingly  represented  ill  the  collections 
of  Carboniferous  fossils  from  Utah  and  Nevada.  Indeed,  all  told,  there 
have  been  but  few  species  recognized  from  the  Paleozoic  of  the  Great 
Basin.  Most  of  those  collected  at  Eureka  are  new  species,  described  for 
the  first  time,  but  allied  to  forms  found  in  the  Mississippi  Valley  and  At- 
lantic States,  while  others  appear  to  be  identical  with  well  known  species 
A  complete  catalogue  of  the  Lamellibranchiates  will  be  found  under  the 
lists  of  Devonian  and  Carboniferous  species  in  an  appendix  at  the  end  of 
this  volume. 

Commingling  of  Carboniferous  Species.— Prof.    R.    P.    Wllitfield    aild    Dr.    C.     A. 

White  have  frequently  called  attention  to  the  commingling  of  Lower  Car- 
boniferous and  Coal-measure  species  in  New  Mexico,  Colorado  and  Utah 
which,  in  the  Mississippi  Valley,  are  quite  distinct  and  regarded  as  char- 
acteristic of  one  or  the  other  of  the  two  horizons.  So  far  as  known  to 
the  writer  nowhere  is  this  commingling  of  types  more  strikingly  brought 
out  than  at  Eureka.  Moreover,  here  they  are  associated  with  species 
which,  in  New  York  and  Ohio,  are  regarded  as  typical  of  the  Devonian, 
several  of  them  being  restricted  within  a  very  limited  vertical  range. 
This  grouping  of  fossils  is  found  on  a  low  hill  on  the  west  base  of 
Spring  Hill,  a  long  monotonous  ridge  lying  just  to  the  east  of 
the  Hoosac  fault  and  made  up  wholly  of  Lower  Coal-measure  strata. 
The  beds  of  Spring  Hill  Ridge,  along  the  fault,  for  the  most  part  dip 
toward  the  east.  On  a  small  but  prominent  outlying  hill  on  the  western 
slope  of  the  ridge  they  lie  inclined  toward  the  west,  the  result  of  an  anti- 
clinal fold  within  the  main  body  of  limestone.  In  this  outlying  hill  occurs 
a  well  marked  bed  of  arenaceous  limestone  dipping  about  50°  to  the  west 
towards  the  Hoosac  fault  and  cropping  out  both  on  the  east  and  west  slopes 
of  the  hill;  the  same  bed  being  recognized  in  the  main  ridge  on  the 
opposite  side  of  the  anticline.  This  limestone,  which  has  been  traced  for 
short  distances,  both  north  and  south,  has  furnished  a  most  varied  fauna. 
Owing  to  its  paleoutological  importance,  Mr.  Walcott  has  given  especial 
attention  to  the  group  and  has  distinguished  over  fifty  forms,  most  of  which 
he  has  specifically  determined.  About  one-third  of  them  he  regards  as  ideu- 


MINGLING  OF  CARBON J  FERGUS  SPECIES.  89 

tical  with  species  found  in  the  Mississippi  Valley  in  Lower  Carboniferous 
rocks,  while  many  of  them  have  usually  been  considered  as  restricted  to 
that  horizon.  Associated  with  them,  in  sufficient  force  to  sho\v  ;i  comming- 
ling of  types,  occur  characteristic  Coal-measure  fossils  like  Athyrix  subtilita 
and  Euomphalus  subrugosus.  Mingled  with  these  fossils,  in  the  same  strata, 
are  the  Lamellibranchiates,  which  present  so  striking  a  feature  of  the  Carbon- 
iferous fauna.  Notwithstanding  the  fact  that  the  Devonian,  at  Eureka, 
furnishes  an  exceptionally  rich  fauna  in  Lamellibranchiates,  nearly  all  the 
species  found  in  the  Carboniferous  occur  for  the  first  time  at  this  horizon, 
and  but  few,  if  any,  specifically  agree  with  the  Devonian  forms.  This  is  all 
the  more  noticeable  because  species,  which  are  identical  with  those  found 
in  New  York  and  Ohio,  are  in  the  latter  localities  only  recognized  in  restricted 
areas xand  inmost  instances  from  horizons  low  down  in  the  Devonian.  This 
is  well  shown  by  the  species  Grammysia  arcuata  and  Macrodon  hamiltonte, 
both  regarded  as  typical  of  the  Hamilton  group,  while  others  like  Sanguino- 
lites  ceolus  is  referred  to  the  Chemung  and  to  the  Waverly  sandstone  of 
Ohio. 

The  complete  list  of  species  from  these  strata  is  as  follows : 

Archseocidaris,  sp.  f  Aviculopecten,  sp.  ! 

Fenestella  (3  sp.  !)  Myalina  nessus. 

Discina  newberryi.  Pterinopecten  hoosacensis. 

Streptorhynchus  crenistria.  Pterinopecten  spio. 

Grthis  resupinata.  Crenipecten  hallanus. 

Chonetes  grannlifera.  Ptychopteria  protoformis. 

Chonetes  verneuiliana.  Pinna  consimilis. 

Productus  prattenianus.  Pinna  inexpectans. 

Productus  semireticulatus.  Modiomorpha  ambigua. 

Spirifera  camerata.  Modioiuorpha  !  desiderata. 

Spirifera  neglecta.  Nucula  insularis. 

Spiriferina  kentuckiensis.  Nucula,  sp.  ? 

Athyris  subtilita  ?  Solenomya  curta. 

Ehynchonella  eurekensis.  Macrodon  truucatus. 

Rhynchonella  (Leiorhynchus  type).  Grainmysia  arcuata. 

Aviculopecteu  affinis.  Grammysia  liannibaleiisis. 

Aviculopecten  eurekensis.  Edinondia  medoii. 

Aviculopecten  haguei.  Sanguinolites  a?olus. 

Aviculopecten  peroccidens.  Sanguinolit«'s  a'olus.  var. 


90  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

Sanguinolites  naeuia.  Euomphalus  subrugosus. 

Sanguinolites  retusus.  Pleurotomaria  nodomarginata. 

Sanguinolites  salteri.  Bellerophon  textilis. 

Sanguinolites  simplex.  Naticopsis,  sp.  ? 

Sanguinolites  striata.  Dentalium,  sp.  f 

Microdon  connatus.  Orthoceras  randolphensis. 

Schizodus  cuneatus.  Ortboceras,  sp.  ? 

Schizodus  deparcus.  Gomphoceras,  sp.  ? 

Cardiola  filicostata.  Griffithides  portlocki. 

Below  this  horizon  there  is  a  bed  of  bluish  gray  limestone  interesting 
on  account  of  its  grouping  of  Lower  Coal-measure  fossils  without  the 
presence  of  any  of  those  species  which  might  be  regarded  as  indicating  a 
lower  stratigraphical  position,  but  which  are  here  found  in  the  overlying 
strata.  The  list  is  small,  but  characteristic  of  the  Coal-measures.  It  is  as 
follows : 

Fenestella,  sp.  f  Productus  semireticulatus. 

Streptorhynchus  crenistria.  Spirifera  camerata. 

Chonetes  granulifera.  Rhynchonella  eurekensis. 

Productus  prattenianus.  Griffithides  portlocki. 

Richmond  Mountain  Fauna.— There  is  some  reason  to  believe  that  the  inter- 
calated arenaceous  and  calcareous  strata  lying  at  the  base  of  the  great 
limestone  belt  all  the  way  from  Richmond  Mountain  southward  to  Fish 
Creek  Valley  represents  a  portion  of  the  chocolate-colored  clay  shales 
underlying  the  limestone  of  Diamond  Peak,  and  referred  to  the  upper 
members  of  the  Diamond  Peak  quartzite.  From  the  base  of  the  Lower 
Coal-measure  limestone  along  the  Hoosac  fault  up  to  the  capping  of 
andesite  lavas  of  Richmond  Mountain  the  highly  inclined  strata  measure 
about  1,800  feet.  Fossils  occur  scattered  throughout  the  limestones.  From 
highly  fossiliferous  strata  favorable  for  their  preservation,  a  grouping  of 
species  was  found  which  may  be  taken  as  typical  of  the  entire  epoch, 
although  only  in  a  few  localities  is  the  life  so  full  and  well  represented. 
This  list  from  the  southwest  base  of  Richmond  Mountain  is  as  follows : 

Zaphrentis.  Streptorhynchus  crenistria. 

Fenestella,  sp.  t  Chonetes  granulifera. 

Lingula  mytaloides.  Productus  longispinus. 

Discina  newberryi.  Productus  nebrascensis. 


WEBER  CONGLOMERATE.  91 

Productus  pratteuianus.  Athyris  hirsuta. 

Productus  semireticulatus.  Rhynchonella  eurekensis. 

Spirifera  annectans.  Camarophoria  cooperensis. 

Spirifera  camerata.  Terebratula  hastata. 

Spirifera  ieidyi.  Aviculopecten  afflnis. 

Spirifera  neglecta.  Streblopteria  similis. 

Spirifera  rockymontana.  Myalina  congeneris. 

Spirifera  atriata.  Bellerophon,  sp.  f 

Spirifera  (M.)  setigera.  Metoptoma  peroccidens. 

Syringothyris  cuspidatus.  Griffithides  portlocki. 

Along  Carbon  Ridge  the  limestones  are  well  developed  but  have  as  yet 
yielded  little  calling  for  special  comment  as  regards  the  life  of  the  period. 
The  limestone  forming  the  top  of  Diamond  Peak  and  the  long  Alpha  ridge 
west  of  Hayes  Canyon  carry  several  fossiliferous  strata  at  different  hori- 
zons, but  all  of  them  present  much  of  the  same  grouping  of  species.  Near 
the  summit  of  Diamond  Peak  a  shaly  limestone  was  found  to  contain 

Polypora  (like  P.  stragula).  Spirifera  (M.)  setigera. 

Orthis  resupinata.  Athyris  roissyi. 

Productus  nebrascensis.  Athyris  hirsuta. 

Productus  semireticulatus.  Griffithides  portlocki  ? 

Spirifera  trigonalis.  Camarophoria  cooperensis. 

It  seems  hardly  necessary  to  repeat  nearly  similar  lists  from  neigh- 
boring localities  so  long  as  there  appears  to  be  no  marked  change  of  fauna 
with  the  development  of  the  limestones.  Most  of  the  species  obtained 
proved  to  be  specifically  identical  with  those  from  the  limestone  body  of 
Richmond  Mountain  and  Carbon  Ridge  east  of  the  Hoosac  fault.  The 
region  of  Diamond  Peak  does  not  offer  as  many  species,  but  on  the  other 
hand  it  has  not  been  as  diligently  searched.  The  Lamellibranchiate  fauna 
was  nowhere  recognized  in  the  region  of  Diamond  Peak. 

Weber  Conglomerate.— Conformably  overlying  the  Lower  Coal-measures 
comes  the  Weber  conglomerate,  one  of  the  most  persistent  and  well  defined 
horizons  over  wide  areas  of  the  Cordillera,  stretching  westward  all  the  way 
from  the  Front  Range  in  Colorado  to  the  Eureka  Mountains.  It  varies  in 
the  nature  of  the  sediment  with  every  changing  condition,  but  it  is  nearly 
everywhere  easily  recognized  as  a  siliceous  formation  between  two  great 
masses  of  Carboniferous  limestone.  In  places  it  is  made  up  of  an  admixture 


92  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

of  calcareous  and  sandy  beds;  in  others,  of  fine  grits  and  shales;  and,  again, 
of  nearly  pure  siliceous  sediment,  varying  from  fine  to  coarse  grained,  de- 
pendent largely  upon  the  distance  from  any  land  area  and  depth  of  water  in 
which  it  was  deposited.  Here  at  Eureka  the  material  is  exceptionally  coarse 
with  abundant  evidence  of  shallow  water  deposition  and  the  existence  of  a 
land  surface  not  very  far  removed  at  the  time  the  beds  were  laid  down. 

Two  large  bodies  represent  the  Weber  conglomerate  at  Eureka,  one 
directly  east  of  Carbon  Ridge  and  the  other  overlying  the  Alpha  Ridge 
west  of  Hayes  Canyon.  The  former  is  not  shown  in  its  full  development, 
the  upper  members  being  cut  off  by  the  Pinto  fault,  but  the  geological 
position  of  the  latter  is  admirably  brought  out  by  the  underlying  and  over- 
lying limestones.  Across  their  broadest  development  the  beds  have  a 
thickness  estimated  at  about  2,000  feet.  They  are  well  shown  in  long  par- 
allel ridges  inclined  at  high  angles,  with  a  synclinal  followed  by  an  anti- 
clinal fold.  For  the  most  part  the  formation  is  made  up  of  coarse  material 
of  both  angular  and  rounded  fragments  of  red,  brown  and  white  grits, 
together  with  jasper,  brown  horustone,  and  green  cherty  pebbles  firmly 
held  together  by  a  siliceous  cement.  Interstratified  in  the  coarse  material 
are  occasional  beds  of  fine,  yellow  white  sandstone,  which  has  been  used  as 
a  lining  for  the  large  smelting  furnaces  at  Eureka.  In  certain  beds  the 
angular  pebbles  predominate,  and  in  others  the  rounded,  but  in  general  there 
is  a  fair  admixture  of  both  varieties.  Near  the  summit  of  the  horizon  a 
single  belt  of  blue  limestone  comes  in,  which,  however,  in  its  lateral  exten- 
sion, may  not  be  persistent.  Considering  the  thickness  and  nature  of  these 
conglomerates,  they  present  an  exceptionally  uniform  appearance  through- 
out, with  almost  no  shale  and  but  little  limestone.  No  subdivisions  need 
be  drawn.  Although  the  formation  has  yielded  no  fossils,  its  structural 
relations  permit  of  its  being  easily  correlated  with  the  Weber  conglomerate 
of  northern  and  eastern  Nevada.  With  the  coarse  conglomerates  of  the 
Weber  at  Agate  Pass1,  in  the  Cortez  Range,  there  is  the  closest  resemblance ; 
both  areas  must  have  been  near  the  shore  line  of  the  Paleozoic  sea,  in  cen- 
tral Nevada. 

1 U.  S.  Geological  Exploration  of  the  Fortieth  Parallel,  vol.  2,  Descriptive  Geology,  p.  574. 


UPPER   COAL-MEASURES.  93 

Upper  Coal-measures.— Beds  of  this  epoch  are  found  conformably  overlying 
the  Weber  conglomerate,  their  true  geological  position  being  admirably 
shown  at  the  head  of  Hunters  Creek  (atlas  sheet  vm)  in  a  belt  of  lime- 
stone about  one  mile  in  length,  west  of  the  Weber  conglomerate  horizon. 
Both  series  of  rocks  dip  to  the  west  at  high  angles,  the  limestones, 
however,  being  cut  off  by  a  body  of  basalt  which  forms  the  mass  of  Basalt 
Peak  and  the  Strahlenberg.  A  much  larger  body  of  this  limestone  is  found 
forming  the  long  uniform  slope  of  Diamond  Peak,  although  there  its  true 
position  is  obscured  by  longitudinal  faults,  which  in  places  bring  it  in  direct 
contact  with  the  Lower  Coal-measures  and  in  others  it  abuts  unconforma- 
bly  against  the  Weber  conglomerate. 

The  thickness  attained  by  the  rocks  of  this  epoch  is  nowhere  exposed 
in  the  district,  the  overlying  beds  having  either  suffered  removal  by 
denudation  or  else  been  concealed  beneath  flows  of  igneous  rocks.  West  of 
Diamond  Peak  a  number  of  narrow  valleys  cross  the  limestone,  but,  as  the 
inclination  of  the  ridge  coincides  closely  with  the  dip  of  the  beds,  they 
nowhere  reveal  any  considerable  thickness.  The  beds  are  estimated  at  500 
feet.  In  the  northern  and  central  portions  of  the  state  of  Nevada  the  Upper 
Coal-measure  limestones  attain  a  development  of  nearly  2,000  feet.  At 
Moleen  Peak, 'just  south  of  the  Humboldt  River,  they  are  estimated  at  1,800 
feet  in  thickness  where  they  conformably  overlie  a  heavy  deposit  of  con- 
glomerates in  their  essential  features  quite  like  the  Weber  conglomerate  of 
Eureka.  In  the  field  the  Upper  Coal-measures  may  be  distinguished  readily 
from  the  Lower  Coal-measures  by  their  lighter  color  and  greater  preva- 
lence of  fine  grained  beds.  These  colors  are  light  bluish  gray  and  drab, 
the  latter  possessing  a  conchoidal  fracture  and  compact  texture.  These 
compact  limestones  frequently  present  forms  of  erosion  quite  different  from 
the  coarse  grained  and  granular  limestones  of  the  Lower  Coal-measures. 
Throughout  the  horizon  the  limestones  are  interstratitied  with  belts  of  grit 

O  . 

and  siliceous  pebbles,  held  together  by  a  calcareous  cement,  in  which  are 
intercalated  thin  beds  of  purer  limestone.  One  or  two  prominent  beds  are 
apparently  made  up  of  quartz  pebbles  and  fragments  (if  an  older  limestone. 
carrying  such  fossils  as  Fiisilina  riilindriru  and  Pi-«<ln<-tnx 


>U.  S.  Geological  Exploration  of  the  Fortieth  Parullol,  vol.  2.   Desrriptive  ecology,  p.  (500. 


94  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

as  if  indicating  that  they  had  been  derived  from  the  underlying  Carbon- 
iferous rocks.  The  fossils,  however,  which  are  all  Coal-measure  species, 
might  be  derived  quite  as  well  from  the  Upper  as  from  the  Lower  beds. 
A  chemical  examination  failed  to  detect  any  beds  of  dolomite  in  the  lime- 
stones, the  highest  amount  of  magnesium  carbonate  obtained  being  T33  per 
cent.  This  is  not  without  interest,  as  it  is  the  only  limestone  horizon  in  the 
Paleozoic  series  at  Eureka  free  from  dolomitic  strata. 

Upper  coal-measure  Fauna.— This  epoch  has  not  yielded  as  large  a  number 
of  species  as  the  Lower  Coal-measures  and  many  of  those  found  in  the 
middle  and  upper  beds  of  the  latter  are  known  to  occur  in  both  divisions  of 
the  Carboniferous  limestone  throughout  the  Great  Basin.  The  following 
list  comprises  all  those  species  obtained  from  the  Upper  Coal-measures 
which  were  not  observed  in  the  Lower  Coal-measures: 

Zaphrentis,  sp.  ?  Ptilodictya  carbonaria. 

Polypora,  sp.  ?  Ptilodictya  serrata. 

Orthis  pecosi.  Productus  punctatus. 

Retzia  monnoni.  Macrodon  tenuistriata. 

Terebratula  bovidens.  Pleurotomaria,  sp.  ? 
Myalina  subquadrata. 

A  further  search  of  the  Lower  Coal-measures  might  show  several  of 
these  species,  and  in  other  localities  outside  the  District  it  is  by  no  means 
certain  that  they  have  not  been  found  lower  down.  Terebratula  bovidens  is 
known  to  range  throughout  the  Coal-measures;  on  the  other  hand  Productus 
punctatus,  a  common  form,  seems  to  be  restricted  to  the  Upper  Coal-meas- 
ures in  the  Great  Basin. 

A  narrow  belt  of  yellowish  gray,  somewhat  shaly,  limestone,  near  the 
head  of  Hunter  Creek,  carries  the  following  grouping  of  fossils : 

Fusilina  cyliudrica.  Productus  semireticulatus. 

Fusilina  robusta.  Spirifera  camerata. 

Chaetetes,  sp.  ?  Spirifera  rockymoiitana. 

Orthis  pecosi.  Spiriferina  cristata. 

Productus  longispinus.  Athyris  subtilita. 

Productus  nebrascensis.  Terebratula  bovidens. 

Productus  prattenianus.  Myalina  subquadrata. 

Productus  punctatus.  Pleurotomaria  (like  P.  turbiiiitbrmis). 


CAKBONIFEKOUS  COAL.  95 

At  the  extreme  northern  end  of  the  district,  on  the  west  slope  of  Diamond 
Peak  and  north  of  Garden  Creek,  in  a  very  similar  limestone,  the  beds 
yielded  as  follows: 

Fusilina  cyliiidrica.  Productus  prattenianus. 

Chaetetes,  sp.  f  Productus  semireticulatus. 

Zaphrentis  (fragments).  Spiriferina  cristata. 

Ptilodictya  (Stenopera)  carbonaria,  !  Athyris  subtilita. 

Ptilodictya  (Stenopera)  serrata,  !  Eetzia  mormon i. 

A  complete  list  of  fossils  from  the  somewhat  restricted  fauna  of  the 
Upper  Coal-measures  will  be  found  at  the  end  of  this  volume. 

Carboniferous  Coal.— In  the  first  range  to  the  east  of  the  Eureka  District, 
Carboniferous  formations  extend  for  miles  along  the  edge  of  the  valley 
which  in  a  study  of  Paleozoic  rocks  present  some  points  of  more 
than  ordinary  interest.  It  is  the  only  range  in  the  Great  Basin  where  coal 
of  Carboniferous  age  has  been  discovered  in  anything  like  a  well  defined 
seam  of  sufficient  thickness  to  encourage  exploration,  although  beds  carry- 
ing small  amounts  of  carbonaceous  matter  are  known  in  one  or  two  other 
localities  in  central  Nevada.  Two  outcrops  of  this  coal  are  known  and 
considerable  exploration  has  been  undertaken  in  order  to  determine  the  value 
of  the  coal  seams ;  one  is  situated  on  a  low  flat  hill  known  as  Pancake  Ridge, 
and  the  other  on  Bald  Mountain,  which  stands  out  prominently  at  the  southern 
end  of  the  Humboldt  Range.  Pancake  lies  about  eight  miles  to  the  west- 
ward of  the  Eureka  District  in  a  mass  of  low  ridges  connecting  the 
Humboldt  Range  with  the  White  Pine  Mountains.  Rising  above  the  plain 
occurs  a  body  of  rhyolite,  beyond  which  is  a  low  ridge  of  coarse  conglom- 
erate followed  by  a  second  ridge  somewhat  higher  than  the  first  with  an 
intervening  valley  or  shallow  depression.  Along  the  western  base  of  this 
second  ridge  an  exposure  of  drab  clay  shales  crops  out  only  a  few  feet  in 
thickness,  striking  approximately  north  and  south  with  a  low  dip  to  the 
east  rarely  exceeding  10°.  This  clay  carries  a  seam  of  lignite  varying 
from  10  to  18  inches  in  width  which  may  be  readily  traced  for  nearly  !.">(> 
feet  along  the  line  of  outcrop.  Both  above  and  below  this  coal  seam  are 
alternating  layers  of  bituminous  shale  and  purer  day  shale  ronformably 
resting  upon  a  bed  of  coarse  conglomerate.  Above  tin-  rlay  shales  comes 


96  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

a  bed  of  conglomerate  about  25  feet  in  thickness  made  up  mainly  of  rounded 
quartz  pebbles  followed  by  another  belt  of  shale  quite  like  the  one  below, 
40  feet  in  thickness.  In  both  series  of  shale  occur  beds  of  carbonaceous 
material  and  thin  seams  of  impure  coal,  but  nowhere  on  the  surface  are  the 
exposures  more  than  three  inches  in  width.  Still  higher  up  is  another  belt 
of  conglomerate  carrying  more  or  less  lime  and  followed  by  buff  colored 
massive  limestone  changing  to  brownish  gray  limestone  followed  by  a  cherty 
limestone,  the  latter  extending  to  the  top  of  the  mountain.  This  series  of 
limestones  has  an  estimated  thickness  of  nearly  1,000  feet.  Fossils  charac- 
teristic of  the  Coal-measures  are  common  throughout  the  limestone,  but  are 
more  abundant  in  the  lower  beds,  more  especially  in  those  immediately 
above  the  coal,  although  no  horizon  presents  any  special  faunal  peculiarities. 
Scattered  throughout  the  limestone  occur  the  following  species: 

Zaphrentis  centralis,  1.  Productus  costatus 

Diphyphyllum,  sp.  I.  Productus  semireticulatus. 

Chaetetes,  n.  sp.  Spirifera  camerata. 

Discina,  sp.  ?.  Spirifera  rockymontana. 

Orthis  pecosi.  Spiriferina  kentuckiensis. 

Qrthis  resupinata.  Retzia  mormcnii. 

Streptorhynchus  crenistria.  Athyris  roissyi. 

Chonetes  grauulifera.  Athyris  subtilita. 

Productus  cora.  Terebratula  bovidens. 

This  grouping  may  be  said  to  present  some  distinctive  features  con- 
taining forms  regarded  as  belonging  to  the  Lower  Carboniferous,  mingled 
with  others  typical  of  the  Coal-measures.  Zapl/rattix  mitm/ix,  Dipliyphyllum 
and  Athyris  roissyi  give  to  the  horizon  a  Lower  Carboniferous  aspect, 
while  the  relatively  large  number  of  Coal-measure  species  would  ordinarily 
determine  the  position  of  the  beds.  Not  only  do  the  Coal-measure  species 
outnumber  the  others,  but  several  of  them  happen  to  be  those  forms  like 
Orthis  pecosi  and  Reteia  mormoni,  which  have  as  yet  been  recognized  only 
in  the  Upper  Coal-measures.  Nevertheless,  the  evidence  of  the  fauna  is 
strongly  in  favor  of  the  lower  horizon  for  these  coal  beds,  as  certain  species 
are  elsewhere  unknown  higher  up  in  the  Carboniferous,  whereas  it  is  a 
feature  of  the  Coal-measure  fauna  of  the  Great  Basin  that" it  presents  a  wide 
vertical  range. 

Lithologically  the  evidence  is  not  specially  decisive.     The  series  of 


BALD  MOUNTAIN  COAL.  97 

beds  at  Pancake  bear  some  resemblance  to  the  section  found  at  the  base 
of  Richmond  Mountain,  which,  of  course,  indicates  the  base  of  the  Carbon- 
iferous limestone.  Such  evidence,  however,  is  not  conclusive,  as  the  beds 
also  resemble  and  may  be  synchronous  with  the  interstratified  grits  and  lime- 
stones of  the  Upper  Coal-measures,  with  which  by  far  the  greater  number  of 
the  observed  species  are  identical. 

In  exploring  these  coal  seams  for  marketable  coal  considerable  work 
has  been  done,  although  all  operations  had  been  abandoned  three  years 
previous  to  our  visit.  In  places  the  vein  was  reported  as  5  feet  in 
width,  although  much  broken  up  and  displaced.  A  vertical  shaft,  said  to 
be  180  feet  in  depth,  had  been  sunk  before  the  project  was  abandoned  and 
several  tunnels  and  inclines  run  along  the  line  of  the  coal.  Examinations 
could  be  made  only  in  one  tunnel,  owing  to  the  caving-in  of  the  clay  beds. 
Sixty  feet  from  the  entrance,  where  the  seam  measures  20  inches,  samples 
of  coal  were  collected.  It  closely  resembles  the  lignites  of  the  Green  River 
basin.  On  exposure  the  coal  crumbles  readily. 

Bald  Mountain  lies  to  the  north  of  Pancake  and  is  situated  in  the 
main  ridge  of  the  Humboldt  Range.  The  coal  or  lignite  outcrops  are  ex- 
posed near  the  base  of  the  range  in  clay  shales  inclined  at  low  angles 
toward  the  mountain.  The  mode  of  occurrence  bears  the  closest  resem- 
blance to  the  strata  at  Pancake — interstratified  conglomerates  and  shales 
followed  by  massive,  distinctly  bedded  yellowish  brown  and  buff  lime- 
stones. At  the  time  of  our  visit,  in  the  autumn  of  1880,  the  Bald  Mountain 
Coal  Company  had  run  a  tunnel  from  the  outcrop  for  160  feet  into  the 
mountain  following  the  coal  seam.  At  the  head  of  this  tunnel  the  coal 
strata  measured  only  from  2  to  7  inches  in  width,  passing  into  black  car- 
bonaceous clays.  At  this  point  there  was  more  or  less  displacement  of 
the  strata,  and  this  thin  seam  of  coal  was  apparently  cut  off  by  a  line  of 
faulting,  which  put  an  end  to  further  explorations,  the  poor  quality  and 
limited  quantity  of  the  coal  discouraging  any  further  outlay  of  money.  A 
search  of  the  black  shale  beneath  the  coal  was  rewarded  by  the  rinding  of 
a  number  of  fossils  all  belonging  to  the  species  Athiiri*  xxbtilita.  In 
the  buff  limestones,  immediately  above  the  coal,  a  small  number  of  fossils 

were  found: 

MON  xx 7 


98 


GEOLOGY  OP  THE  EUEEKA  DISTRICT. 


Orthis  pecosi.  Spirifera  rockymontana. 

Streptorhyuchus  creriistria.  Athyris  subtilita. 

Productus  cora.  Eetzia  mormoiii. 

Productus  setnireticulatus. 

They  represent  a  distinctively  Coal-measure  fauna  and  are  identical 
with  forms  collected  from  beds  at  Pancake.  On  the  other  hand,  none  of 
the  species  obtained  at  Pancake,  indicating  the  horizon  at  the  base  of  the 
Lower  Coal-measures,  have  as  yet  been  found  at  Bald  Mountain.  In  this 
grouping  at  Bald  Mountain  there  is  nothing  to  prevent  the  horizon  from 
being  considered  as  belonging  to  the  Upper  Coal-measures,  but  it  is  hardly 
possible  to  suppose  that  the  geological  position  of  these  beds  differs  from 
the  position  of  the  coal  at  Pancake.  The  geological  mode  of  occurrence 
at  both  places  and  the  sections  across  the  beds  indicate  that  the  coal  comes 
from  near  the  same  horizon,  and  was  deposited  under  similar  conditions,  with 
the  probabilities  in  favor  of  their  having  at  one  time  formed  a  continuous 
coal  area. 

The  following  analyses  of  samples  of  these  coals,  collected  at  the  time 
of  our  visit,  are  given  here  for  the  purpose  of  showing  the  character  of  the 
deposits.  They  were  made  by  Dr.  W.  F.  Hillebrand,  of  the  U.  S.  Geolog- 
ical Survey. 


No.  1, 
Pancake. 

No.  2, 
Bald  Moun- 
tain. 

Moisture 

6-17 

2-60 

Volatile  matter  

31-88 

30-97 

Fixed  carbou  ..       ..  

55-59 

44-60 

Ash  

6-36 

21-83 

Total 

100-00 

100-00 

Sulphur  in  pvrites 

0-73 

5-44 

Sulphur  in  soluble  sulphates  

0-79 

0-14 

The  coals  do  not  cake  or  sinter. 

These  coals,  while  they  are  of  no  commercial  value,  are  of  geological 
importance  from  their  exceptional  mode  of  occurrence  in  the  Carboniferous 
rocks  of  the  Great  Basin.  A  further  search  would  doubtless  indicate  whether 
they  belong  to  the  base  of  the  Lower  Coal-measures  or  to  the  middle  of 
the  Upper  Coal-measures. 


CHAPTER   V. 
DESCRIPTIVE  GEOLOGY. 

In  the  following  pages  will  be  found  a  detailed  description  of  the  sed- 
imentary rocks  in  the  Eureka  District,  the  order  followed  being  for  the  most 
part  the  same  as  that  adopted  in  the  chapter  devoted  to  the  general 
geological  sketch.  Each  orographic  block  is  described  by  itself,  beginning 
with  Prospect  Ridge,  where  the  oldest  rocks  occur,  followed  by  the  other 
blocks  according  to  the  geological  succession  of  strata;  the  only  changes 
made  in  the  order  of  treatment  being  for  the  purpose  of  bringing  out  more 
forcibly  the  structural  relations  of  the  individual  blocks  to  each  other.  This 
chapter  necessarily  contains  a  repetition  of  many  facts  stated  in  other  por- 
tions of  the  volume,  but  at  the  same  time  there  is  an  omission  of  many 
details  that,  if  not  presented  elsewhere,  would  properly  find  a  place  here. 

The  principal  object  of  this  chapter  is  to  give  a  connected  description 
of  the  country  and  to  place  numerous  details  in  permanent  form  for  the 
use  of  those  who  may  wish  to  study  the  field  in  person,  or  who  may  desire 
to  investigate  more  fully  the  facts  upon  which  the  generalizations  are  based. 
Certain  portions  of  the  country  are  described  more  fully  than  others,  and 
in  a  few  instances  the  descriptions  follow  closely  those  given  elsewhere  in 
the  volume. 

PROSPECT   RIDGE   REGION. 

This  region  includes  all  the  country  lying  between  the  Hoosac  fault 
on  the  east  side  and  the  Spring  Valley,  Prospect  Mountain,  and  Sierra 
faults  on  the  west.  These  lines  of  faulting  sharply  outline  a  mountain 
block  which,  in  its  geological  structure,  stands  out  on  all  sides  clearly 
defined  from  the  adjacent  country. 

99 


100  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

Along  the  east  side  of  the  Hoosac  fault  no  sedimentary  rocks  are 
known  other  than  those  belonging  to  the  Lower  Coal-measures,  while  on 
the  west  side  of  the  other  three  faults  only  Silurian  and  Devonian  beds  are 
brought  up  against  the  fault  line.  In  this  uplifted  mountain  mass  lying 
between  these  great  lines  of  faulting  occur  all  the  Cambrian  rocks  exposed 
in  the  district,  with  the  exception  of  two  small  patches  of  limestone,  one  of 
Prospect  Mountain  limestone  and  one  of  Hamburg  limestone,  found  on  the 
west  side  of  Surprise  Peak,  one-half  mile  to  the  westward  of  the  line  of 
the  Sierra  fault.  They  occur  in  a  region  of  much  local  disturbance,  not  far 
from  the  body  of  hornblende-andesite  which  occupies  the  bottom  of  Sierra 
Valley,  and  are  of  no  special  geological  interest  otherwise  than  indicating 
great  displacement  of  strata.  This  uplifted  mass  of  Prospect  Ridge  meas- 
ures 10  miles  in  length  by  about  2\  milesin  width  across  its  broadest  expan- 
sion, in  the  region  of  Prospect  Peak,  in  places  narrowing  to  one-half  that 
distance.  Within  this  block,  evidence  of  minor  fractures  and  dislocations 
are  everywhere  to  be  seen,  influencing  in  a  greater  or  less  degree  the  geo- 
logical structure  of  the  country. 

jackson  Fault.— Two  faults,  designated  as  the  Jackson  and  the  Ruby 
Hill,  profound  in  their  displacement  and  of  great  economic  importance, 
deserve  special  mention ;  both  of  them,  however,  lie  within  the  limits  of  the 
Prospect  Ridge  uplift.  The  Jackson  fault  starts  in  just  north  of  the  Eureka 
tunnel,  in  Goodwin  Canyon,  on  the  east  side  of  the  ridge,  and  may  be 
traced  northward  along  the  line  of  contact  between  Prospect  quartzite  and 
the  Hamburg  limestone  (atlas  sheet  vin).  It  follows  down  the  narrow  ravine 
past  the  Jackson  mine  to  the  east  of  Ruby  Hill  and  Adams  Hill,  and  is  lost 
near  the  body  of  quartz-porphyry  just  beyond  the  Wide  West  ravine.  This 
fault  brings  up  the  Pogonip  limestone  of  the  Silurian  against  the  entire 
series  of  Cambrian  strata  of  Ruby  Hill,  from  the  lower  quartzite  to  -the 
Hamburg  shales  inclusive.  On  the  east  side  of  this  fault  exploration  has 
failed  to  bring  to  light  any  large  and  permanent  bodies  of  ore,  if  we  except 
that  of  the  Williamsburg  mine ;  the  more  valuable  mining  properties  in  the 
immediate  neighborhood  of  Ruby  Hill  being,  for  the  most  part,  on  the  west 
side  of  the  fault  in  the  Cambrian  rocks. 


PKOSPEOT  KIDGE  SECTION.  101 

Ruby  Hill  Fault.— This  fault  starts  in  near  the  reservoir  in  New  York 
Canyon,  branching  out  from  the  Hoosac  fault  and  running  in  a  northwest 
direction.  It  cuts  diagonally  across  the  Pogonip  limestone,  and  abruptly 
terminates  the  Hamburg  limestone  and  Hamburg  shale,  which  form  such 
persistent  topographic  features  of  the  country  to  the  southward,  and  inter- 
sects the  Jackson  fault  near  the  American  shaft,  just  south  of  the  Jackson 
mine  (atlas  sheet  vm).  For  a  short  distance  this  fault  apparently  coincides 
with  the  Jackson  fault,  then  crosses  it,  following  a  northwesterly  direction — 
the  same  course  it  held  before  the  intersection  with  the  great  north  and  south 
fault.  On  Ruby  Hill  the  fault  may  be  traced  in  the  underground  workings 
of  all  the  principal  mines  through  to  the  Albion.  It  has  exerted  a  most 
powerful  influence  upon  the  structure  of  Ruby  Hill,  and  from  its  relations 
to  the  ore  bodies  its  importance  from  a  mining  point  of  view  can  not  be 
overestimated.  Reference  will  be  made  to  this  fault  in  the  discussion  on 
the  geology  of  Ruby  Hill. 

From  Spring  Valley  eastward  across  Prospect  Ridge  and  Hamburg 
Ridge  to  the  Hoosac  fault,  the  highly  inclined  strata  offer  an  unbroken 
geological  section  from  the  lowest  beds  of  the  Cambrian  to  the  Eureka 
quartzite  of  the  Silurian.  It  offers  the  best  section  to  be  found  in  Nevada 
of  the  Cambrian  rocks,  with  all  the  epochs  into  which  it  has  been  divided 
clearly  denned.  Sections  across  Prospect  Mountain  limestone  may  vary 
greatly  in  details  within  a  few  hundred  feet  in  the  relative  thickness  of 
compact  limestone  and  calcareous  shaly  beds,  but  in  general  the  sections 
across  the  entire  thickness  of  the  horizon  coincide  fairly  well. 

Section  CD-EF  (atlas  sheet  xui),  constructed  across  the  central  portion 
of  the  Eureka  Mountains,  intersects  Prospect  Ridge  about  3,000  feet  to  the 
north  of  the  peak,  at  a  point  selected  to  bring  out  the  anticlinal  structure  of 
the  mountains.  The  underlying  quartzite  is  overlain  on  both  sides  of  the 
fold  bv  the  Prospect  Mountain  limestone,  which  on  the  west  side  extends 
down  to  Spring  Valley,  while  on  the  opposite  side  it  forms  not  only  the 
summit,  but  the  entire  east  wall  of  the  main  ridge.  This  is  in  turn  over- 
lain by  the  remaining  subdivisions  of  the  Cambrian,  all  of  wlxich  stand 
inclined  at  a  uniformly  high  angle  to  the  east.  As  the  section  is  drawn 
across  a  high  saddle  at  the  head  of  New  York  Canyon,  connecting  Pros- 


102 


GEOLOGY  OF  THE  EUREKA  DISTRICT. 


pect  Peak  and  Hamburg  Ridge,  the  erosion  of  the  Secret  Canyon  shale  is 
not  so  well  shown  as  it  would  be  if  the  section  had  been  drawn  either  to  the 
north  or  south  of  this  point,  but  it  is  quite  sufficient  to  bring  out  the  promi- 
nence of  the  Hamburg  Ridge,  which  is  everywhere  parallel  to  the  main 
ridge.  Overlying  the  Hamburg  shale  occurs  the  Pogonip  limestone,  in  turn 
followed  by  the  Eureka  quartzite,  which  occupy  the  long  slope  down  to 
the  Hoosac  fault.  The  entire  series  of  beds  dips  to  the  east,  with  angles 
varying  from  75°  to  85°.  The  section  across  these  beds  from  the  axis  of 
the  fold  is  as  follows  : 

Prospect  Ridge  Section. 


Feet. 

Feet. 

500 

Com  actvitreou    whit            t  i       '  d'    '       bedd' 

500  • 

Massive  siliceous  dark  pray  limestone,  occasionally  black  lime- 

550 

2,150 

Fim-  grained,  evenly  bedded,  ash  gray  limestone,   with  more 

1.250 

Calcareous  shales,  passing  into  thin-bedded  limestones;  bands 

350 

350 

{Yellow  argillaceous  shale,  with  thin  layers  of  gray  limestone; 

350 

1,200 

(  Dark  gray  granular  limestone;  only  slight  traces  of  bedding; 

)     in  places  highly  siliceous;  beds  brecciated  in  the  upper  portion. 
f  Argillaceous  shales,  yellow  and  brown  in  color  

1.200 

750 

1,600 

100 

750 

Massive,   light  gray  limestones,  passing  into  bluish  gray  and 
bluish  black  beds,  with  occasional  bands  of  black  limestone 

1,250 

Fissile  calcareous  shales,  with  a  thin  band  of  green  and  drab- 

350 

Prospect  Mountain  limestone  

3,050 

700 

Argillaceous  shales,  ash  gray  in  color;  weathering  red  and  yel- 

350 

Light  gray  compact    limestone,    with    thin  seams  of   ealcite 

300 

100 

Prospect  Mountain  quartzite 

1  000 

{Bedded  vitreous  quartzite;    weathering   dark    brown;    inter- 

1  000 

Total  thickness  

9  350 

Along  the  line  of  this  section  there  has  been  less  faulting,  crushing 
and  local  displacement  than  anywhere  else  on  the  ridge.  Such  local 
disturbance  as  has  taken  place  in  the  uplifted  mass  is  more  apparent  in  the 
Prospect  Mountain  limestone  than  in  the  other  horizons,  partly  owing  to 


PEOSPECT  EIDGE  SECTION.  103 

frequent  changes  in  the  physical  conditions  of  the  alternating  beds  of  shale 
and  limestone  and  partly  to  the  fact  that  this  series  of  beds  forms  the  sum- 
mit of  the  ridge  and,  lying  nearer  the  axis  of  the  fold,  has  been  subjected 
to  much  greater  pressure  and  strain.  The  shales,  yielding  easily  to  pressure, 
have  folded  and  flexed  under  excessive  strain,  while  the  more  compact 
limestones,  under  the  same  force,  were  faulted  and  fissured.  Evidence  of 
this  is  seen  in  the  Mountain  shale  belt  and  the  overlying  limestone,  the 
former  exhibiting  a  tendency  to  flatten  out  and  the  latter  to  recover  the 
normal  dip  by  a  sharp  break,  causing  numerous  fissures  and  faults.  Since 
the  first  uplifting  of  the  mountain,  intrusive  dikes  of  rhyolite  have  filled 
preexisting  fissures  and  broadened  lines  of  weakness,  besides  causing  addi- 
tional faulting  and  displacement.  These  intrusive  masses,  however,  are 
for  the  most  part  narrow  and  have  produced  no  fundamental  structural 
changes,  but  much  of  the  secondary  alterations,  such  as  local  meta- 
morphism  of  beds,  the  cementation  of  brecciated  limestone,  and  similar 
phenomena,  are  easily  explained  by  their  action. 

Numerous  tunnels,  run  for  the  purpose  of  mining  exploration,  vary- 
ing from  50  feet  to  several  hundred  feet  in  length,  penetrate  the  Prospect 
Mountain  limestone  all  along  the  ridge,  at  different  elevations.  Among 
them  may  be  mentioned  the  Fourth  of  July,  Maryland,  Lemon,  and 
Golden  Era  tunnels.  Most  of  them,  however,  extend  only  for  short  dis- 
tances, and,  while  they  offer  fair  sections  of  portions  of  the  great  limestone 
belt  and  may  have  subserved  the  purposes  of  the  miner,  are  of  but  little 
value  for  purposes  of  geological  structure.  Two  tunnels,  the  Eureka  and 
Prospect  Mountain,  running  at  right  angles  to  the  strike  of  the  beds  and 
from  opposite  sides  of  the  ridge,  give  admirable  sections  across  nearly  the 
entire  thickness  of  the  limestone  belt. 

Eureka  Tunnel.— The  entrance  to  the  Eureka  tunnel  is  situated  near  the 
head  of  Goodwin  Canyon,  to  the  west  of  the  Hamburg  Ridge  (atlas  sheet 
vm).  The  tunnel  starts  in  near  the  base  of  the  Hamburg  limestone  and 
is  driven  in  a  nearly  due  west  direction  for  2,000  feet,  passing  several 
hundred  feet  beyond  the  crest  of  the  ridge  and  about  800  feet  below. 
The  following  is  the  series  of  beds  encountered  in  the  Eureka  tunnel, 
beginning  at  the  entrance: 


104  GEOLOGY  OF  THE  EUBEKA  DISTRICT. 

Feet. 
Black  crystalline  limestone  (Hamburg  limestone) 85 

Argillaceous  shale  (Secret  Canyon  shale) 300 

(Prospect  Mountain  limestone) : 

Limestone 935 

Calcareous  shale 30 

Brecciated  limestone 51 

Mountain  shale 460 

Stratified  limestone 90 

Brecciated  limestone 50 

The  body  of  limestone  near  the  entrance  to  the  tunnel  belongs  to  the 
base  of  the  Hamburg  limestone  and  is  a  small  mass  left  by  erosion 
upon  the  west  side  of  Goodwin  Canyon,  the  canyon  for  the  most  part  having 
been  eroded  along  the  line  of  contact  between  the  Hamburg  limestone  and 
the  Secret  Canyon  shale.  Where  the  tunnel  enters  the  mountain  the  Secret 
Canyon  shale  pinches  out  to  a  few  hundred  feet,  and,  a  short  distance  to  the 
north,  it  is  entirely  cut  off  by  the  Prospect  Mountain  quartzite.  At  the 
tunnel  the  shales  are  only  300  feet  in  thickness.  Through  the  Prospect 
Mountain  limestone  nearly  all  signs  of  stratification  and  bedding  are  want- 
ing, the  rocks  everywhere  showing  evidence  of  crushing  and  local  faulting. 
Evidence  of  movement  is  seen  in  the  brecciated  appearance  of  the  lime- 
stone, which  has  been  recemented  by  calcite.  Fissures  and  seams 
nearly  vertical  are  common,  dipping  slightly  both  to  the  east  and 
west;  the  larger  number  of  them  being  inclined  toward  the  east.  Dyna- 
mic action  has  caused  such  frequent  changes  throughout  the  limestone 
that  it  is  difficult  to  recognize  any  belt  by  lithological  distinctions.  The 
narrow  bed  of  shale,  30  feet  in  thickness,  is  a  well  defined  belt,  calcareous, 
and  more  or  less  argillaceous,  but  of  little  importance,  simply  fore- 
shadowing the  coming  in  of  the  broad  belt  of  Mountain  shale  beyond. 
Whether  it  would  be  found  to  be  continuous  on  further  exploration,  either 
to  the  north  or  south,  is  questionable.  Beyond  this  narrow  shale  band 
occurs  another  limestone  belt,  similar  to  the  main  body,  in  turn  followed  by 
the  Mountain  shale,  which,  unlike  the  Secret  Canyon  shale,  is  character- 
ized by  intercalated  limestone.  It  resembles  the  clay  shale  found  on 
the  surface,  but  is  less  pure  than  the  Secret  Canyon  body.  It  bears  a 
close  resemblance  to  the  shale  belt  found  in  the  Prospect  Mountain  lime- 
stone of 'Ruby  Hill,  but  there  is  no  direct  evidence  of  their  ever  having 


PEOSPECT  MOUNTAIN  TUNNEL.  105 

formed  a  continuous  bed.  Nowhere  else  on  the  ridge  do  the  Mountain 
shales  appear  so  broadly  developed,  300  feet  being  the  greatest  thickness 
observed  on  the  surface.  Beyond  this  shale  belt  the  limestone  is  occasion- 
ally stratified  and  then  again  occurs  crashed  and  broken,  showing  that  it 
has  undergone  much  pressure;  the  stratified  rock  in  general  lying  next  the 
shale. 

From  a  geological  point  of  view  the  value  of  the  tunnel  lies  in  the  evi- 
dence of  the  crushing,  faulting  and  fissuring  which  the  entire  series  of  beds 
have  undergone  since  the  first  uplift  of  the  mountain,  the  changes  in  the 
character  of  the  limestone  being  far  better  studied  in  the  tunnel  than  on  the 
surface.  A  marked  fissure,  slightly  inclined  to  the  east,  occurs  about  840  feet 
from  the  mouth  of  the  tunnel.  Stringers  of  ore,  or  rather  indications  of 
ore,  are  encountered  all  through  the  limestone,  but  few  of  them  are  of 
economic  value,  being  mainly  filled  with  calcite,  oxide  of  iron  and  man- 
ganese and  carrying  but  little  lead  and  silver.  At  one  point  a  nearly  per- 
pendicular pipe  connects  with  the  surface,  but  carries  no  ore.  A  small 
amount  of  ore  was  discovered  near  by,  however,  just  north  of  the  tunnel. 
The  largest  body  of  ore  opened  by  the  tunnel  occurs  nearly  1,200  feet  from 
the  entrance,  the  metal-bearing  fissure  running  approximately  north  and 
south  and  standing  nearly  vertical.  At  the  time  of  our  visit  this  was  the 
only  ore  body  encountered  which  was  of  sufficient  economic  value  to  be 
profitably  worked ;  but  since  then  a  fair  amount  of  good  ore  has  been 
extracted. 

Prospect  Mountain  Tunnel.— This  mining  tunnel  starts  in  at  the  west  base 
of  Prospect  ridge  at  an  elevation  of  about  7,200  feet  above  sea  level 
(atlas  sheet  vn).  It  has  been  driven  about  2,350  feet  into  the  moun- 
tain, with  a  course  a  little  north  of  west,  but  does  not  penetrate  quite  to 
the  center  of  the  ridge,  the  slope  of  the  mountain  being  more  gradual  on 
the  west  than  on  the  east  side;  if  prolonged  it  would  pass  the  crest  of  the 
mountain  only  a  few  hundred  feet  south  of  the  Eureka  tunnel.  It  lies 
wholly  in  the  Prospect  Mountain  limestone,  which,  being  less  fractured  and 
brecciated  than  the  limestone  toward  the  east,  offers  a  more  typical  cross 
section,  although  there  is  but  little  well  defined  bedding.  For  the  first 
100  feet  from  the  entrance  the  tunnel  passes  through  a  dark  gray  rock, 
beyond  which  it  becomes  much  lighter  in  color  and  apparently  uniform  in 


106  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

texture  for  500  feet.  From  this  point  frequent  belts  of  crystalline  white 
marble  occur,  alternating  with  compact  light  gray  limestone.  Specimens 
in  the  collection  show  a  very  fair  qiiality  of  marble.  A  marked  change  in 
the  limestone  comes  in  about  1,500  feet  from  the  entrance,  where  a  fissure 
is  met  at  right  angles  to  the  tunnel,  inclined  a  few  degrees  to  the  west  from 
the  vertical;  beyond  this  point  the  character  of  the  limestone  more  closely 
resembles  the  brecciated  rock  found  on  the  east  side  of  the  ridge,  as  shown 
in  the  Eureka  tunnel.  This  resemblance  is  borne  out  by  the  appearance 
of  a  belt  of  stratified  limestone,  followed  by  argillaceous  shale  like  the 
Mountain  shale,  but,  as  the  latter  occurs  at  the  head  of  the  tunnel  and  has 
not  been  fully  explored,  its  true  position  is  unknown;  it  may  simply  be 
one  of  the  many  lenticular  shale  bodies  observed  elsewhere  in  the  Prospect 
Mountain  limestone.  One  or  two  fissures  were  cut  by  the  tunnel,  but  little 
ore  was  found,  the  most  promising  indication  of  an  ore  body  being  worked 
for  a  short  time  without  any  profitable  return.  At  475  feet  from  the 
entrance  there  is  a  well  defined  fissure  connecting  with  the  surface,  suffi- 
ciently large  to  admit  light  and  air.  It  evidently  at  one  time  formed  a 
drainage  channel  for  surface  waters,  as  is  shown  by  the  smoothly  rounded, 
water-worn  sides.  The  Eureka  and  Prospect  Mountain  tunnels  nearly 
pierce  the  ridge,  the  two  taken  together  being  over  four-fifths  of  a  mile  in 
length. 

charter  Tunnel.— The  Charter  tunnel  lies  mainly  in  the  Prospect  Mountain 
quartzite.  The  entrance  is  situated  in  the  drift  deposits  of  Spring  Valley, 
just  west  of  Mineral  Hill,  but  soon  after  enters  the  quartzite,  which  here 
forms  the  western  base  of  the  ridge  as  it  rises  above  the  valley.  In 
1882  it  had  a  total  length  of  700  feet,  with  a  trend  of  N.  64°  W.,  affording 
a  good  exposure  across  the  beds.  This  tunnel,  where  it  cuts  the  quartzite 
south  of  Ruby  Hill,  exposes  narrow  bands  of  highly  altered  rock,  com- 
posed of  fine  siliceous  material  associated  with  monoclinic  pyroxene  and 
pyrites.  On  the  ridge  above  the  tunnel,  and  not  far  below  the  overlying 
limestones,  occurs  a  band  of  exceedingly  fine-grained  rock,  light  green  in 
color  and  made  up  of  an  aggregation  of  quartz,  monoclinic  pyroxene, 
white  in  thin  section,  probably  diopside,  and  glossularite,  a  lime  garnet 
In  the  ravine  immediately  south  of  Ruby  Hill  is  a  small  body  of  iron 


MAGNETIC  ORE.  107 

ore,  which  analysis  shows  to  be  magnetite.  It  possesses  some  interest 
from  its  position  in  the  lower  Cambrian  rocks,  but  on  account  of  the  lim- 
ited amount  is  of  no  economic  value.  Material  dried  at  104°  C.  yielded 
Mr.  J.  E.  Whitfield  the  following  result: 

Per  cent. 

Silica 5-29 

Titanic  acid None 

Sulphuric  acid -36 

Alumina  -18 

Ferric  oxide 64-69 

Ferrous  oxide 18-96 

Mangauous  oxide 1-16 

Lime -88 

Magnesia 5-X5 

Water..  2-Bx 


Total  100-25 

Prospect  Ridge.— North  of  the  Prospect  and  Eureka  tunnels  the  main  ridge 
loses  its  simple  anticlinal  structure  and  a  synclinal  fold,  much  distorted  and 
broken,  takes  its  place.  From  about  the"  line  of  these  tunnels  to  the  northern 
end  of  Mineral  Hill  it  is  difficult  to  make  out  the  structural  features.  The 
Prospect  quartzite,  which  is  obscured  for  some  distance  by  the  overlying 
limestone,  reappears  again  along  the  west  base  of  the  ridge,  curves  around 
on  the  north  side  of  the  small  body  of  granite  exposed  at  the  north  end  of 
Mineral  Hill,  and  may  be  traced  southward  on  the  east  side  of  Prospect 
Ridge  in  a  continuous  body  until  terminating  abruptly  near  the  Eureka 
tunnel,  where  it  is  cut  off  by  a  fault;  its  eastern  extension  is  determined  by 
the  sharp  line  of  the  Jackson  fault.  Overlying  the  quartzite  comes  the 
Prospect  limestone,  forming  the  summit  of  Mineral  Hill,  with  lines  of 
bedding,  although  much  obscured,  dipping  into  the  ridge  on  both  sides  of 
the  hill.  By  reference  to  atlas  sheet  vu,  the  synclinal  structure  of  Mineral 
Hill  may  be  readily  understood,  the  quartzite  coming  in  along  the  base  of 
the  hill  on  both  sides,  with  the  limestone  crushed  and  broken  occupying  the 
crest  of  the  ridge. 

That  the  small  granite  body  at  the  northern  end  of  Mineral  Hill,  directly 
opposite  Ruby  Hill,  exerted  an  influence  in  determining  the  structure  of 
Prospect  Ridge,  seems  evident,  but  in  just  what  manner  it  is  difficult  to 


108  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

say.  The  relation  of  this  granite  to  the  Prospect  Mountain  uplift  will  be 
more  fully  considered  in  discussing  the  geology  of  Ruby  Hill. 

South  of  Prospect  Peak  the  limestone  maintains  a  fairly  persistent 
north  and  south  strike  and  easterly  dip,  the  angle  of  which  seldom  falls 
below  60°.  These  highly  inclined  beds  occur  for  a  long  distance  north  of 
the  Geddes  and  Bertrand  mine.  In  the  Irish  Ambassador  the  beds  lie 
inclined  at  40°.  In  general,  lines  of  bedding  have  been  obliterated,  but 
are  found  in  sufficient  number  of  instances  to  establish  the  structure,  while 
a  meager  fauna  affords  ample  evidence  of  the  age  of  the  beds.  Near  the 
Geddes  and  Bertraud  mine  in  a  compact  limestone,  the  upper  horizons  of 
the  Prospect  Mountain  limestone  are  identified  by  the  occurrence  of  several 
species  found  also  in  the  Richmond  Mine  on  Ruby  Hill,  as  well  as  by  other 
forms  found  in  the  same  belt  just  below  the  Secret  Canyon  shale.  These 
beds  yielded  Kutorgina  whitfieldi,  Plychoparia  oweni,  and  Agnostus  bidcns. 
Lenticular  beds  of  argillaceous  shale  are  by  no  means  as  broadly  developed 
as  to  the  northward,  but  are  of  frequent  occurrence  and  indicate  the  same 
alternating  conditions  of  deposition.  On  the  other  hand  cherty  beds  and 
highly  siliceous  dark  limestones  are  very  characteristic  of  the  region. 
Occasionally  thin  siliceous  beds,  from  their  superior  hardness,  withstanding 
erosion  better  than  the  purer  beds,  rise  like  walls  above  the  surrounding 
hill  slopes.  This  latter  feature  frequently  gives  the  limestone  body  quite  a 
different  aspect  from  that  observed  to  the  north  and  at  the  same  tune  aids 
in  determining  the  strike  of  the  beds. 

As  already  mentioned  the  Eureka  quartzite  on  the  west  side  of  the 
Sierra  fault"  lies  unconformably  against  the  Prospect  Mountain  limestone 
from  Prospect  Peak  nearly  to  Surprise  Peak.  At  this  latter  locality  a  body 
of  Pogonip  limestone  abuts  against  the  Cambrian  limestone;  the  fault  line, 
which  has  maintained  a  persistent  direction,  swerves  suddenly  eastward  and 
then  again  turns  and  with  a  north  and  south  course  strikes  across  an  easterly 
spur  of  Surprise  Peak.  On  a  broad  shoulder  of  this  spur  the  Prospect 
Mountain  limestone  again  comes  in  contact  with  the  Eureka  quartzite  of 
Surprise  Peak,  the  line  of  faulting  passing  about  200  feet  below  the  summit. 
Structurally  the  position  of  the  Pogonip  limestone  is  shown  by  its  passing 
conformably  beneath  the  Eureka  quartzite.  Paleontological  evidence  con- 


DRAINAGE  OF  SECRET  CANYON.  109 

firms  this  fact  by  the  finding  of  a  group  of  Silurian  fossils  which  are 
characteristic  of  the  upper  beds  of  the  horizon.  Among  the  species  found 
here  on  the  north  base  of  the  Peak  are  Ortiiis  perveta,  0.  tricenaria,  Itaphis- 
toma  nasoni,  and  Eeceptaculites  mammillaris.  The  Prospect  Mountain  lime- 
stone follows  around  on  the  south  side  of  Surprise  Peak,  thence  southward 
until  lost  beneath  the  extravasated  lavas,  which  encircle  the  ridge  where  it 
falls  away  toward  Fish  Creek  valley.  From  Surprise  Peak  southward 
these  limestones  lie  unconformably  against  Pogonip  beds,  the  former  stand- 
ing at  the  usual  high  angles  of  60°  or  more,  and  the  latter  also  dipping 
eastward,  but  at  angles  varying  from  35°  to  45°. 

secret  Canyon.— This  canyon  forms  one  of  the  most  prominent  physical 
features  of  the  district,  a  deeply  eroded  valley  lying  between  two  parallel 
ridges,  one  of  Prospect  Mountain  limestone  and  the  other  of  Hamburg 
limestone.  The  canyon  lends  its  name  to  the  intermediate  body  of  argil- 
laceous shales  which  are  better  exposed  here  than  elsewhere.  For  more 
than  2  miles  in  length  the  narrow  valley  is  cut  out  of  these  easily  eroded 
beds,  the  harder  limestones  rising  upon  each  side  in  abrupt  walls  several 
hundred  feet  in  height.  There  are  few  finer  instances  to  be  found  any- 
where of  a  valley  carved  out  of  soft  friable  material,  the  beds  of  which 
lie  highly  inclined  and  conformable  with  overlying  and  underlying  strata 
of  superior  hardness,  withstanding  erosion  better.  No  one  overlooking 
Secret  Canyon  from  any  high  point  in  the  country  would  understand  the 
appropriateness  of  the  appellation;  its  true  significance  is  recognized  only 
when  approached  from  the  south.  The  course  of  the  present  drainage 
channel  follows  the  trend  of  the  shales  until  nearly  opposite  the  southern 
end  oi  Roundtop  Peak,  when,  instead  of  maintaining  its  direction  along 
the  line  of  the  shales  for  a  short  distance  further  and  thence  out  through 
the  Quaternary  covered  slopes  to  Fish  Creek  valley,  it  turns  suddenly,  fol- 
lows a  narrow  defile  obliquely  through  the  ridge  of  Hamburg  limestone  and 
shale,  carves  its  way  through  the  Pogonip  and  Eureka  quartzite,  cn> 
the  Hoosac  fault,  and  is  again  deflected  to  the  south  only  by  Carbon  Ridge. 
The  reason  for  its  leaving  the  valley  of  Secret  Canyon  is  to  be  found  in 
the  rhyolite  mass  which  probably  underlies  the  hills  of  detritus  near  the 
entrance  to  the  canyon,  blocking  the  former  drainage  channel.  This  is, 


110  GEOLOGY  OF  THE  EUKEKA  DISTRICT. 

however,  only  a  partial  explanation,  as  it  is  difficult  to  understand  why  the 
stream  should  not  continue  on  its  course,  cutting  its  way  through  the  low 
rhyolite  barrier,  rather  than  turn  io  the  east  and  follow  the  present  course, 
which  it  finally  took  across  the  uplifted  sedimentary  beds.  There  seems 
no  doubt  that,  before  the  rhyolite  eruption,  the  stream  bed  followed  the 
canyon  and  emptied  directly  into  Fish  Creek  Valley. 

Of  the  shale  formation,  little  need  be  said  in  addition  to  the  descrip- 
tions already  given  of  the  beds.  They  show  great  uniformity  of  deposi- 
tion and  physical  character,  monotonous  in  outline  and  color,  and,  so  far  as 
recognized,  carry  no  organic  remains.  The  sandy,  limy  transition  strata 
into  the  Hamburg  limestone  generally  offers  better  lines  of  stratification 
than  either  the  shales  below  or  the  limestones  above,  and  the  dip  and  strike 
may  be  determined  at  a  number  of  points  along  the  base  of  the  overlying 
horizon. 

Hamburg  Ridge.— Along  the  east  side  of  Secret  Canyon  the  Hamburg 
limestone  and  shale  and  the  Pogonip  limestone  horizons  form  a  single  ridge, 
which,  although  of  less  elevation  and  of  less  rugged  aspect,  is  singularly 
like  Prospect  Ridge  in  its  salient  topographical  features.  With  the  excep- 
tion of  the  summit  of  Roundtop,  all  the  more  elevated  portions  are  found 
in  the  Hamburg  limestone.  Although  evidences  of  bedding  are  for  the 
most  part  obliterated  in  the  Hamburg  limestone,  they  are  by  no  means  so 
exceptional  as  to  leave  any  doubt  that  the  ridge  dips  easterly  with 
great  uniformity.  Occasional  beds  are  found  with  a  dip  and  strike  not  in 
accordance  with  this  general  structure,  but  in  such  instances  they  can  be 
shown  to  be  the  results  of  local  disturbance  produced  by  the  action  of  in- 
trusive rhyolites.  In  studying  the  district,  care  has  been  taken  to  discrim- 
inate between  such  local  disturbances,  which  may  be  very  considerable 
within  limited  areas,  and  the  structure  due  to  the  primary  upheaval  and  the 
blocking  out  of  the  great  mountain  masses.  At  the  southern  end  of  the 
ridge  the  Hamburg  limestone  has  been  a  good  deal  broken  up  under  the 
influence  of  the  rhyolites  of  Gray  Fox  and  the  numerous  small  dikes 
of  the  same  intrusive  rock.  Here  the  beds  are  seen  standing  nearly 
vertical,  sometimes  inclined  westerly,  and  again  resuming  the  normal 
dip  to  the  east.  The  limestone  beds  throughout  are  highly  siliceous. 


ROUNDTOP  MOUNTAIN.  HI 

Black  cherty  bands  and  beds  of  black  quartzite  form  a  characteristic 
feature  of  the  horizon.  One  of  these  siliceous  beds  on  the  crest  of  the 
ridge  may  be  followed  for  a  long  distance  without  any  break  in  the  con- 
tinuity and  is  sufficiently  well  marked  to  form  a  characteristic  feature  of 
the  ridge. 

Evidence  of  the  age  of  these  beds,  based  upon  their  organic  remains, 
rests  mainly  upon  the  material  obtained  from  the  limestone  immediately 
overlying  the  Secret  Canyon  shale.  Fossils  are  known  to  occur,  more  or 
less  well  preserved,  in  a  number  of  places,  but  the  most  satisfactory  locali- 
ties are  found  just  north  of  the  Geddes  and  Bertrand  dike  and  immediately 
west  of  the  divide  separating  Secret  Canyon  from  New  York  Canyon.  All 
the  species  obtained  are  identical  with  those  collected  from  the  same  hori- 
zon north  of  Ruby  Hill.  Midway  up  the  west  slope  of  Hamburg  Ridge, 
and  nearly  due  west  from  Roundtop,  several  species  with  much  the  same 
grouping  occur  in  a  dark,  compact  limestone — a  locality  which,  if  thor- 
oughly examined,  might  possibly  yield  a  rich  fauna.  The  Hamburg  shale 
forms  a  well  marked  horizon,  but,  being  harder  and  more  compact,  yields 
less  readily  to  erosion,  and,  in  consequence,  is  less  easily  determined  by 
topographical  features  than  the  same  horizon  northward.  It  may  be 
traced  from  the  extreme  southern  end  of  the  ridge  northward  across  the 
broad  west  spur  of  Roundtop,  until  abruptly  cut  off  by  the  rhyolite  body 
which  occupies  Glendale  Valley.  The  Pogonip  limestone  has  much  the 
same  north  and  south  limits,  rising  gradually  out  of  the  rhyolitic  tuffs  at 
the  base  of  Gray  Fox  Peak  on  the  south,  and  terminating  in  a  high  wall 
which  forms  the  west  side  of  the  upper  Glendale  Valley. 

Roundtop  Mountain.— Roundtop  Mountain  is  almost  wholly  made  up  of 
Pogonip  limestone,  and  offers  the  best  exposure  of  the  series  of  beds 
characteristic  of  this  horizon  to  be  found  in  the  southern  part  of  the  Eureka 
District.  On  the  spur  running  out  to  the  west  from  the  top  of  the  moun- 
tain, and  in  an  arenaceous  limestone  immediately  above  the  Hamburg 
shale,  a  few  organic  remains  were  obtained,  belonging  to  a  characteristic 
grouping  which  marks  the  transition  from  Cambrian  to  Silurian,  found  in 
several  other  localities  at  the  base  of  the  Pogonip.  On  the  southern  spur 
of  Roundtop,  in  beds  dipping  from  65°  to  70°  eastward,  a  small  but 


112  GEOLOGY  OF  THE  EUEEKA  DISTRICT. 

characteristic  fauna  occurs,  in  which  were  found  Lingula  manticula,  Orthis 
hamburgensis,  0.  testudinaria,  Tiplesia  calcifera,  and  Ptychopatria  liaguei.  To 
the  north  of  Roundtop  the  beds  are  much  broken  up  by  volcanic  masses, 
the  structure  being  most  difficult  to  make  out  and  the  beds  impossible  to 
follow,  but  beyond  this  again  the  beds  recover  their  normal  position,  strik- 
ing north  and  south  and  dipping  at  a  high  angle  to  the  east,  until  the  entire 
series  of  beds  is  lost  beneath  the  rhyolite.  Along  the  east  slope  of  Round- 
top  the  Eureka  quartzite  dips  generally  eastward,  an  exception  being  the 
block  lying  between  Glendale  Valley  and  the  ravine  coming  down  from  the 
north  slope  of  Roundtop.  Here  it  has  been  thrust  violently  forward  toward 
the  south  and  dips  with  a  high  angle  to  the  southwest,  in  marked  contrast 
to  the  main  body. 

Along  the  west  slope  of  Hoosac  Mountain  both  the  Hamburg  shale  and 
the  Pogonip  limestone  again  come  to  the  surface,  the  latter  rising  within 
200  feet  of  the  top  of  the  mountain,  the  line  between  the  two  limestones 
being  defined  as  elsewhere  by  the  occurrence,  although  poorly  preserved, 
of  a  grouping  of  species  characteristic  of  the  border  line  between  the  Cam-- 
brian  and  the  Silurian. 

Hoosac  Mountain.— This  bold  mountain  mass,  situated  to  the  east  of  the 
Hamburg  Ridge,  attains  an  elevation  several  hundred  feet  higher  than  any 
point  along  the  ridge,  rising  prominently  above  the  immediate  country  with 
an  altitude  of  over  8,500  feet  above  sea  level.  The  broad  summit  for 
nearly  one-half  mile  in  length  maintains  approximately  the  same  elevation, 
a  few  points  here  and  there  rising  slightly  above  the  general  level.  With 
the  exception  of  the  narrow  strip  of  Pogonip  limestone  upon  the  west  slope, 
the  Eureka  quartzite  forms  the  entire  mountain.  The  mountain  falls  off 
gradually  to  the  north  and  south,  but  more  or  less  abruptly  to  the  east, 
where  the  quartzite,  broken  down  by  a  series  of  small  parallel  faults, 
presents  numerous  low  walls  and  cliffs  toward  the  Hoosac  fault.  The 
quartzite  body,  where  it  is  possible  to  determine  any  structure,  trends  inva- 
riably north  and  south  and  dips  easterly,  but  nothing  can  be  made  out  as  to  its 
thickness,  owing  to  the  great  amount  of  local  displacement.  The  quartzite 
resembles  the  horizon  as  seen  elsewhere,  except  that  it  is  more  or  less  altered 
by  solfataric  action  and  by  the  intrusive  rocks,  which  penetrate  it  as  narrow 


HOOSAC   MINE.  113 

dikes.  There  occur  here  some  curious  bands  of  a  dark  brecciated  quartzite 
made  up  of  chert  and  jasper,  in  fragments  firmly  cemented  together  and 
brilliantly  colored  by  secondary  alteration.  The  cementation  probably  fol- 
lowed the  infiltration  of  silica,  which  took  place  during  the  volcanic  period. 
Both  hornblende-andesite  and  rhyolite  penetrate  the  mountain,  but  mainly 
in  narrow  dikes,  the  surface  exposures  of  which  are  much  decomposed  and 
in  most  instances  so  altered  as  to  render  a  study  of  them  impossible ;  no 
dikes  of  perfectly  fresh  rock  were  observed.  Miners  searching  for  ore 
bodies  along  the  outcrops  of  these  decomposed  rocks  have  explored  them  in 
a  way  to  permit  of  their  general  course  and  mode  of  occurence  being  made 
out.  From  underground  exploration  there  is  reason  to  believe  that  but  a 
small  part  of  the  andesite  dikes  reach  the  surface,  and  these  only  in  stringers 
and  offshoots  from  some  parent  body.  Mapping  the  hornblende-andesite 
exposures  along  the  mountain,  they  are  seen  to  follow  a  common  course 
approximately  north  and  south,  coincident  with  the  lines  of  faulting  and 
the  trend  of  the  mountain  uplift,  following  the  direction  of  the  main 
Hoosac  fault.  Although  much  decomposed,  the  andesitic  character  of 
these  rocks  can  be  readily  made  out  from  a  study  of  their  hornblendes  and 
glassy  feldspars;  the  latter  under  the  microscope  are  found  to  be  all  tri- 
clinic.  The  rhyolite  exposure  just  east  of  the  Hoosac  mine  appears  to  be 
a  remnant  left  by  erosion  from  the  main  body  of  the  Hoosac  fault  outburst. 

The  Hoosac  mine,  situated  on  the  east  slope  of  the  mountain,  is  one 
of  the  oldest  mining  properties  in  the  district,  having  been  located  in  1869 
and  opened  early  the  succeeding  year.  As  it  is  the  only  mine  in  the  dis- 
trict found  in  the  Eureka  quartzite,  it  has  much  geological  interest,  and  its 
development  has  served  at  least  to  furnish  data  bearing  upon  the  structure 
of  a  singular  mountain.  A  vertical  shaft  200  feet  in  depth  has  been  sunk 
through  the  quartzite,  from  the  bottom  of  which  a  level  300  feet  in  length 
runs  westward  into  the  mountain.  All  the  mine  workings  lie  in  quartzite, 
the  ore  bodies  encountered  being  found  in  connection  with  the  intrusive 
rocks.  It  is  reported  that  the  owners  of  the  property  took  out  in  a  short 
time  precious  metals  to  the  value  of  $500,000.  Continued  exploration 
failed  to  maintain  the  high  hopes  first  entertained  of  the  mine. 

Northward  of  Hoosac  Mountain  the  Pogouip  limestone  maintains,  as 
MONXX 8 


114  GEOLOGY  OF  THE  EUEEKA  DISTRICT. 

far  as  New  York  Canyon,  its  uniform  and  simple  structure,  while  the 
Eureka  quartzite,  on  the  other  hand,  occurs  only  here  and  there  in  irregular 
patches  cropping  out  from  beneath  heavy  flows  of  hornblende-andesite, 
which  come  to  the  surface  along  the  line  of  the  Hoosac  fault.  This  profound 
fault  coming  up  from  the  south  may  be  said  to  bifurcate  at  New  York  Can- 
yon, the  main  branch  swerving  off  to  the  northeast,  retaining  the  name  of 
Hoosac  fault,  the  other,  trending  to  the  northwest,  being  designated  as 
the  Ruby  Hill  fault.  Between  these  two  lines  of  faulting  lies  a  block  of 
uplifted  beds,  which  in  structure  is  in  some  respects  quite  independent  of 
the  Prospect  Mountain  Ridge,  a  result  probably  brought  about  by  the 
dynamic  forces  which  produced  both  the  Ruby  Hill  and  Jackson  faults 
and  the  rhyolite  outbursts  of  Purple  Mountain.  This  block  is  wholly  made 
up  of  Silurian  strata,  all  three  periods  being  represented.  The  Ruby  Hill 
fault  may  be  traced  on  the  surface  from  New  York  Canyon  to  its  junction 
with  the  Jackson  fault  by  the  numerous  outbursts  of  rhyolite,  whereas 
northward  along  the  Jackson  fault  no  rhyolite  has  anywhere  been  observed. 
As  far  north  as  Shadow  Canyon  the  strata  incline  southwest  toward 
McCoy's  Ridge,  but  beyond  this  canyon  the  dip  and  strike  of  the  beds  is 
most  irregular,  in  general  dipping  away  from  the  Jackson  fault  and  under 
Purple  Mountain  and  Caribou  Hill.  The  greatest  thickness  of  limestones 
anywhere  represented  in  this  belt  is  about  2,700  feet,  measured  across  the 
strata  from  Shadow  Canyon  to  McCoy's  Ridge.  The  age  of  the  limestone 
underlying  the  quartzite  of  McCoy's  Ridge  is  determined  by  the  presence 
of  a  Pogonip  fauna  characteristic  of  the  upper  horizons,  serving  also  to 
identify  the  quartzite  which  here  forms  such  a  persistent  ridge  along  the 
north  side  of  New  York  Canyon.  The  trend  of  the  ridge  is  determined  in 
part  by  the  direction  of  the  Hoosac  fault  and  in  part  by  the  outbursts  of 
the  lavas  of  Purple  Mountain.  The  limestones  overlying  the  quartzites  can 
be  no  other  than  the  Lone  Mountain  beds.  Although  they  cany  no  organic 
remains,  their  geological  position  and  lithological  habit,  quite  like  the  Lone 
Mountain  strata  immediately  over  the  Eureka  quartzite  elsewhere,  leave  no 
doubt  as  to  their  true  correlation.  It  is  the  only  exposure  of  Lone 
Mountain  limestone  found  in  the  uplift  of  Prospect  Mountain  Ridge,  but 
owing  to  the  want  of  well  denned  lines  of  stratification  no  reliable  estimate 


GEOLOGY  OF  RUBY  HILL.  H5 

can  be  made  of  the  thickness.  There  are,  however,  only  200  or  300  feet 
of  beds  before  the  horizon  is  sharply  cut  on"  bv  the  Hoosac  fault  bringing 
in  the  Carboniferous  in  juxtaposition  with  it. 

Caribou  Hill,  separated  from  McCoy's  Ridge  by  Purple  Mountain,  stands 
out  as  a  prominent  topographical  feature.  It  is  capped  by  the  same 
Eureka  quartzite.  There  are  only  200  feet  of  beds  and  consequently  the 
Lone  Mountain  limestones  are  wholly  wanting.  It  is  this  cap  of  quamite 
which  has  protected  from  erosion  the  underlying  limestones.  Here,  again, 
in  a  narrow  ravine  at  the  west  base  of  the  hill,  in  the  underlying  limestone 
immediately  beneath  the  quartzite,  the  Receptaculites  beds  occur,  with  several 
characteristic  species,  offering  additional  proof,  if  any  was  needed,  as  to  their 
geological  position.  From  Caribou  Hill  northward  no  outcrops  of  the 
Eureka  quartzite  were  recognized.  The  Pogonip  limestones  present  low, 
flat-topped  ridges  inclined  northward,  gradually  passing  beneath  the  recent 
deposits  of  Diamond  Valley. 

RUBY    HILL    REGIOX. 

Ruby  Hill  and  Adams  Hill  together  occupy  a  small  but  clearly  denned 
area  which  may  be  considered  simply  the  northern  extension  of  Prospect 
Ridge.  The  Jackson  fault  sharply  outlines  this  area  on  the  east  side,  while 
the  recent  accumulations  along  the  line  of  the  Spring  Valley  fault  limit  it  on 
the  west  side.  The  geological  importance  of  the  region  is  mainly  derived 
from  the  enormous  ore  deposits  found  in  the  limestones  of  Ruby  Hill,  which 
had  yielded,  up  to  the  time  of  this  investigation,  over  860,000,000  in 
precious  metals.  In  general  the  orographic  structure  is  simple,  and  only  in 
detail  in  the  immediate  neighborhood  of  Ruby  Hill  is  it  in  anv  way  complex. 

On  Plate  i  will  be  found  a  geological  map  of  Ruby  Hill  and  the  adja- 
cent country,  prepared  from  the  large  atlas  sheets  for  more  easy  reference 
to  the  text.  Unfortunately  the  line  between  atlas  sheets  vn  and  viu  runs 
directly  across  this  area,  interfering  greatly  with  the  clear  understanding 
of  the  structural  relations  of  the  beds  of  Prospect  Ridge  with  those  of 
the  Ruby  Hill  as  well  as  with  those  lying  east  of  the  .Jackson  fault.  Hy 
referring  to  the  map  it  will  be  readily  seen  that  the  Jackson  fault  cuts  off 
the  Cambrian  strata  and  brings  the  Pogonip  up  against  the  entire  series. 


116  GEOLOGY  OF  THE  EUEEKA  DISTRICT. 

Granite.— North  of  the  granite  exposure  at  the  end  of  Mineral  Hill  the 
strata  all  dip  northward,  curving  gently  around  the  crystalline  rock  which 
apparently  has  acted  as  a  center  of  upheaving  forces.  The  beds  present  a 
broad  anticlinal  arch,  less  and  less  disturbed  as  they  recede  from  the  granite 
and  with  a  slightly  decreasing  angle  of  dip.  The  granite  body  occupies  but  a 
small  area  on  the  steep  slope  of  Mineral  Hill.  It  is  quite  obscure  in  its  sur- 
face exposure,  and  might  readily  be  overlooked  but  for  its  probable  influ- 
ence in  producing  the  present  geological  features  of  the  country.  Fortu- 
nately, it  gives  some  clue  to  the  peculiarities  of  structure.  The  age  of 
this  granite  is  by  no  means  easily  determined,  but  the  evidence  seems  'to 
show  that  it  was  a  portion  of  an  Archean  island,  around  which  the  sedi- 
ments were  deposited.  At  some  later  period  there  was  a  movement  of  the 
entire  region,  and  the  beds  were  uplifted  and  arched  into  their  present 
position  around  the  granite.  The  exposure  of  the  granite  is  wholly  due  to 
erosion,  and  up  to  quite  a  recent  date  was  covered  with  quartzite.  There  is 
reason  to  believe  that  at  the  time  the  quartzite  was  deposited,  a  land  surface 
existed  at  no  great  distance,  and  this  granite  may  have  been  connected  with 
it.  Evidence  in  favor  of  such  a  supposition  was  found  near  the  bottom  of 
the  Richmond  shaft,  1,200  feet  below  the  surface.  The  vertical  shaft,  after 
passing  through  limestone  as  far  as  the  seventh  level  of  the  mine,  pene- 
trates the  quartzite  for  500  feet.  In  a  white,  fine  grained  quartzite,  small 
pieces  of  rock  were  obtained,  closely  resembling  granite.  Although  some- 
what decomposed,  the  rock  was  found  to  be  made  up  of  quartz,  mica,  and 
an  altered  highly  kaolinized  mineral,  probably  feldspar. 

Encircling  the  granite  and  resting  directly  upon  it,  occurs  the  Prospect 
Mountain  quartzite,  followed  in  turn  by  the  Prospect  Mountain  limestone, 
Secret  Canyon  shale,  Hamburg  limestone,  Hamburg  shale,  and  Pogouip  lime- 
stone, the  entire  series  of  sedimentary  beds  exposed  on  Prospect  Ridge.  That 
the  Ruby  Hill  series  of  beds  were  once  continuous  with  those  of  Prospect 
Ridge  there  is  no  reason  to  doubt,  ample  evidence  being  found  in  the  char- 
acter of  their  sedimentation  and  the  sequence  of  strata.  The  continuity 
was  broken  only  by  profound  faulting  in  much  later  times.  As  the  quartzite 
lies  next  the  granite  it  is  much  broken  up  in  the  sharp  tunis  which  it  is  com- 
pelled to  make  as  the  underlying  rock  of  the  arch.  No  dips  or  strikes  can  be 


0  S  GEOLOGICAL   SURVEY 


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ytJATEKNAKY 


jfiKffiMk 

GEOLOGICAL  MAP  OF  RUBY  HILL 

EUREKA.  MINING  DISTRICT,  NEY 

CAMBRIAN 


Pojjotup 


H«nbui<          3«HT«lCA.      Prospect  IT 

ShaU-  Iim«uone  ShM* 


GEOLOGY  OF  ADAMS  HILL.  H7 

made  out  except  on  the  slopes  of  Ruby  Hill,  where  the  beds  are  distinctly 
seen  to  pass  beneath  the  limestone  which  caps  the  hill.  Owing  to  this 
abrupt  curve,  and  the  consequent  breaking  up  of  the  strata,  erosion  has 
cut  a  deep  ravine  in  the  quartzite.  It  is  this  ravine  which  separates  Ruby 
Hill  from  the  main  ridge.  Overlying  the  quartzite  comes  the  Prospect  Moun- 
tain limestone  forming  the  summit,  the  isolation  of  the  hill  being  made  com- 
plete by  the  erosion  of  a  broad,  shallow  ravine  in  the  Secret  Canyon  shale 
on  the  north  side. 

Adams  Hill,  a  flat  topped  mass  of  Hamburg  limestone,  lies  between 
two  nearly  parallel  ravines,  one  of  which  is  eroded  in  the  Secret  Canyon  or 
underlying  shale,  and  the  other  in  the  overlying  Hamburg  shale.  On  the 
south  side  the  Secret  Canyon  shale  passes  beneath  the  limestone,  the  line 
of  contact  being  well  determined  at  the  base  of  the  hill,  the  dip  and  strike 
of  the  beds  agreeing  closely  with  those  found  on  Ruby  Hill.  On  the  north 
side  of  Adams  Hill  the  Hamburg  shales  appear  and  are  sharply  denned  by 
the  limits  of  the  Wide  West  ravine.  Beyond  this  latter  ravine  the  Pogonip 
limestone  comes  in,  gradually  falling  away  beneath  the  deposits  of  Diamond 
Valley.  On  PL  n,  Sec.  3,  will  be  found  a  geological  section  drawn  across 
the  strata  from  the  Prospect  Mountain  quartzite  on  the  south  slope  of  Ruby 
Hill  to  the  Silurian  limestone,  the  two  Cambrian  limestones  forming  the 
summits  of  the  two  hills,  the  underlying  one  capping  Ruby  Hill  and  the 
overlying  one  forming  the  mass  of  Adams  Hill.  The  section  is  drawn 
across  a  body  of  quartz-porphyry  which  breaks  through  the  Pogonip  lime- 
stone. It  is  quite  unlike  any  other  crystalline  body  known  in  the  district, 
but  it  is  of  no  special  value  as  it  has  exerted  little  influence  upon  the 
limestone,  the  latter  being  very  little  disturbed  and  showing  but  few  signs 
of  alteration.  The  age  of  the  quartz-porphyry  is  unknown,  as  it  penetrates 
Silurian  rocks  only,  but  it  is  probably  older  than  the  rhyolites,  which  it  in 
no  way  resembles  except  in  mineral  composition. 

A  comparison  of  the  section  referred  to  with  the  one  across  Prospect 
Ridge  (atlas  sheet  xm)  brings  out  the  complete  correlation  between  the  two 
series  of  beds,  and  the  great  similarity  in  the  configuration  of  the  two  areas, 
Ruby  Hill  and  Adams  Hill  to  the  north  corresponding  with  Prospect  Ridge 
and  Hamburg  Ridge  of  the  east  and  west  section  of  the  main  mountain. 


118  GEOLOGY  OF  THE  ETJEEKA  DISTEICT. 

On  PI.  n,  Sec.  4,  there  is  shown  for  comparative  purposes  a  section 
across  the  highest  point  of  Prospect  Peak  where  the  quartzite  reaches  the 
very  summit  of  the  ridge.  On  Prospect  Peak  the  strata  stand  at  an  angle 
of  nearly  70°,  whereas  on  Ruby  Hill  and  Adams  Hill  they  lie  inclined  at 
about  40°. 

Paleontological  evidence  that  the  Ruby  Hill  series  of  beds  are  the 
precise  equivalent  of  those  found  011  the  east  side  of  Prospect  Ridge  is 
ample  for  all  purposes  of  identification.  Three  well  defined  horizons  are 
recognized  yielding  the  same  organic  forms  which  characterize  identical 
strata  elsewhere.  The  lowest  of  these  three  horizons  is  found  not  far  below 
the  summit  of  the  Prospect  limestone,  the  middle  one  near  the  base  of  the 
Hamburg  limestone  and  the  upper  one  near  the  base  of  the  Pogonip. 

Fossils  in  Richmond  Mine.— In  a  compact  stratified  limestone  on  the  seventh 
level  of  the  Richmond  Mine  a  sufficient  number  of  organic  forms  were 
found  to  identify  the  beds  with  the  upper  members  of  the  Prospect  Moun- 
tain limestone,  and  locating  beyond  all  question  the  geological  position  of 
the  ore  bodies.  The  species  collected  were: 

Lingula  manticnla.  Agnostus  neon. 

Agnostus  communis.  Agnostus  richmoiidensis. 

Agnostus  bidens.  Ptychoparia  oweni. 

At  the  base  of  the  Hamburg  limestone  opposite  the  Richmond  dump, 
and  again  north  of  the  Albion  mine,  species  have  been  identified  correspond- 
ing to  those  obtained  in  New  York  Canyon  and  Secret  Canyon  just  above 
the  great  shale  body.  North  of  the  Wide  West  ravine  a  small  grouping  of 
forms  correlates  the  limestone  just  above  the  shales  as  the  base  of  the 
Pogonip,  showing  the  mingling  of  the  Cambrian  fauna  with  a  grouping  of 
fossils  which  higher  up  in  the  beds  becomes  characteristic  of  the  Pogonip. 
The  two  species  Obolella  discoidea  and  DiceUocephalvs  marica,  occurring  in  the 
Pogonip  elsewhere,  have  been  collected  from  the  limestones  north  of  the 
Wide  West  ravine. 

FISH    CREEK   MOUNTAINS. 

Fish  Creek  Mountains.— These  somewhat  isolated  mountains  lie  to  the  south- 
west of  Prospect  Ridge.  They  are  surrounded  on  three  sides  by  the  ever- 
present  sagebrush  valleys  of  Nevada,  but  to  the  northward  maintain  their 


ANTICLINAL  STRUCTURE.  H9 

connection  with  the  Eureka  Mountains  by  a  complicated  system  of  ridges 
which  closely  unites  them  with  both  Prospect  Ridge  and  the  Mahogany 
Hills.  Although  their  northern  limit  is  very  ill  denned,  they  stretch  in  a 
north  and  south  direction  for  10  or  12  miles  and  measure  about  5  miles  in 
width,  with  an  elevation  above  the  surrounding  valleys  of  over  2,000  feet. 
Bellevue  and  White  Cloud  Peaks  are  the  two  most  prominent  points  in  the 
mountains,  the  former  with  an  altitude  of  8,883  feet,  the  latter  of  8,850  feet 
above  sea  level,  while  between  them  is  a  still  higher  table-topped  summit, 
having  an  elevation  of  8,951  feet  above  the  sea. 

In  structure  the  main  body  of  Fish  Creek  Mountains  consists  of  an 
anticlinal  fold,  whose  axis  lies  along  the  eastern  edge  of  the  broad,  slightly 
inclined  table  which  forms  the  top  of  the  range.  A  north  and  south  line 
of  faulting  coincides  with  this  axial  plane  and  is  accompanied  by  an  escarp- 
ment, nearly  600  feet  in  height,  showing  a  downthrow  at  least  equal  to 
that  amount.  The  displacement  may  be  traced  readily  for  a  considerable 
distance  along  the  mountain.  The  fault  is  not  laid  down  on  the  map,  but 
the  escarpment  itself  is  indicated  by  the  contour  lines  being  thrown  close 
together.  At  the  base  of  this  cliff"  the  rocks  are  much  broken  up,  as  there 
appears  to  be  a  series  of  small  faults  rather  than  one  sharp  displacement. 
The  anticline  is  nevertheless  sharply  brought  out  by  the  limestone  dipping 
in  opposite  directions  with  a  marked  difference  in  the  angle  of  inclination. 
The  beds  of  the  cliff  incline  at  low  angles  into  the  mountains,  whereas  the 
slopes  upon  the  east  side,  with  an  average  dip  of  15°,  fall  away  abruptly 
for  about  1,500  feet  or  until  buried  beneath  the  Quaternary  deposits  of  Fish 
Creek  Valley.  On  the  west  side  of  the  main  axis  the  limestones  assume  a 
gentle  synclinal  roll,  followed  by  a  low,  broad  anticline,  the  westerly  dip- 
ping beds  of  which  extend  for  nearly  two  miles,  with  a  monotonous  uni- 
form dip,  rarely  exceeding  5°  or  6°,  till  lost  beneath  the  detrital  accumula- 
tions of  Antelope  Valley.  The  geological  structure  is  that  of  a  faulted 
anticline,  gentle  on  one  side  and  relatively  steep  on  the  other,  a 
structure  typical  of  many  ranges  in  the  Great  Basin.  Besides  the  north 
and  south  anticlinal  fold  there  is  a  gentle  quaquaversal  dip  from  the  central 
mass  about  Bellevue  Peak,  the  beds  to  the  northward,  however,  dipping 
away  steeper  than  in  the  other  directions. 


120  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

All  three  divisions  of  the  Silurian  are  found  here — the  Pogonip  lime- 
stone, Eureka  quartzite,  and  Lone  Mountain  limestone.  This  orographic 
block  is  one  of  the  few  mountain  ranges  made  up  wholly  of  Silurian  rocks. 
Nearly  all  the  more  elevated  portions  are  formed  of  Pogonip  beds,  which 
gradually  pass  under  the  overlying  Eureka  quartzite,  which  forms  continu- 
ous bodies  to  the  west  and  north.  The  drainage  channels  running  out  from 
the  summit  are  narrow  ravines,  and,  although  cutting  hundreds  of  feet  into 
the  Pogonip,  never,  so  far  as  is  known,  expose  the  underlying  Cambrian 
strata.  It  is  probable  that  only  the  higher  Pogonip  beds  are  represented. 
Abrupt  walls  of  nearly  black  limestone,  characteristic  of  the  upper  mem- 
bers of  this  horizon,  form  the  sides  of  these  ravines,  in  many  instances  the 
dark  rock  being  capped  by  overlying  beds  of  white  Eureka  quartzite, 
showing  that  these  upper  beds  were  in  place.  This  is  especially  noticeable 
to  the  northwest  of  White  Cloud  where  the  heads  of  nearly  all  the  ravines 
occur  in  the  quartzites.  Near  the  summit  of  the  range  they  cut  through 
nearly  vertical  walls  of  quartzite  from  200  to  400  feet  in  thickness.  Out- 
lying patches  of  quartzite,  remnants  of  erosion,  are  still  to  be  seen  capping 
the  ends  of  the  ridges  on  both  slopes  of  the  mountains.  These  isolated 
patches  are  seldom  more  than  50  feet  in  thickness;  they  lie  scattered  all  over 
the  slopes,  many  of  them  being  so  small  and  obscure  as  to  be  unrepresented 
on  the  map.  Over  the  long  western  slopes  detached  blocks  of  quartzite 
may  be  found  resting  on  the  limestone,  showing  that  while  the  quartzite  has, 
for  the  most  part,  been  carried  away,  the  uppermost  beds  of  limestone  still 
remain  in  place.  The  Receptaculites  beds  extend  in  all  directions  under  the 
quartzite,  paleontology  confirming  structural  evidence  of  their  geological 
position.  All  three  species  of  the  genus  Receptaculites  known  in  the  Great 
Basin  have  been  recognized  here,  associated  with  a  varied  fauna  typical  of 
this  horizon  elsewhere,  with  the  same  foreshadowing  of  Trenton  species. 
The  same  specific  forms  occur  here  that  are  found  underlying  McCoy's 
Ridge  and  Caribou  Hill.  A  list  of  the  species  obtained  at  Bellevue  and 
White  Cloud  Peaks  will  be  found  on  page  53. 

Bellevue  Peak  is  capped  with  Eureka  quartzite  which,  from  here  north- 
ward, stretches  in  a  continuous  body  to  Reese  and  Berry  Canyon.  Over 
this  intermediate  country  it  presents  much  the  same  general  features,  a 


GEANITE-PORPHYET.  121 

white  vitreous  rock  inclined  at  angles  seldom  exceeding  10°  and  frequently 
horizontal.  The  country  offers,  in  places,  broad  table-topped  masses,  and 
;  i  gain  in  others  is  roughly  accidented,  caused  by  numerous  minor  faults 
and  small  displacements,  producing  picturesque  mural-like  cliffs  that  serve 
to  break  the  otherwise  monotonous  scenery.  A  measurement  of  the  thick- 
ness of  the  quartzite  is  impossible.  These  displacements,  although  fre- 
quent, are  seldom  sufficient  to  bring  the  underlying  limestones  to  the  surface. 
The  greatest  thickness  observed  in  any  vertical  wall  is  about  300  feet,  which, 
however,  fails  to  take  into  account  the  amount  carried  off  from  the  surface 
by  denudation.  A  section  across  the  vertical  cliff  just  west  of  Castle 
Mountain  will  be  found  on  page  56.  Near  the  base  of  the  quartzite  cross- 
bedding  has  been  detected  in  one  or  two  localities,  indicating  shallow  water 
deposits;  it  appears,  however,  to  be  wanting  in  all  the  higher  beds  that 
present  a  singularly  uniform  body  of  quartz  grains  free  from  impurities. 

Castle  Mountain  is  capped  by  200  feet  of  Lone  Mountain  limestone 
overlying  the  quartzite,  and  from  here  extends  in  a  narrow  belt  in  a  south- 
east direction  for  over  2  miles.  Here,  as  in  many  other  localities,  the  Lone 
Mountain  limestone  is  devoid  of  fossils,  and  not  until  Stromatopora,  Chcetetes, 
and  Atrypa  reticularis  appear  in  beds  generally  regarded  as  Devonian,  have 
organic  forms  been  recognized.  The  country  is  monotonous  in  the  extreme, 
dazzling  to  the  eyes,  waterless,  and  for  the  most  part  treeless.  The  lime- 
stone shows  no  lines  of  stratification. 

Granite-porphyry.— To  the  northwest  of  Bellevue  and  White  Cloud  Peak, 
in  the  region  of  the  granite-porphyry  dikes,  the  simple  structural  features 
of  the  Fish  Creek  Mountains  are  lost  by  the  intrusion  of  large  bodies  of 
granite-porphyry.  It  occurs  in  two  distinct  masses  with  a  few  outlying 
smaller  dikes  and  knolls,  the  two  principal  bodies  being  separated  by  a 
belt  of  limestone  scarcely  300  feet  in  width. 

The  largest  exposure  of  granite-porphyry  presents  an  irregular  body 
lying  between  Fish  Creek  Mountain  and  Mahogany  Hills  on  the  extreme 
western  edge  of  the  District.  The  smaller  body  occurs  as  a  prominent 
north  and  south  dike,  which,  breaking  through  Pogonip  limestone,  appears 
at  the  surface  as  an  offshoot  from  the  larger  mass.  From  this  massive  dike 


122  GEOLOGY  OF  THE  EUEEKA  DISTEICT. 

several   lesser  ones  branch  off,    nearly  all  of  them  lying  approximately 
parallel  with  the  same  northeast  trend. 

On  the  summit  of  the  Fish  Creek  Mountains,  midway  between  Belle- 
vue  and  White  Cloud  Peaks,  occurs  a  vertical  dike  of  granite-porphyry  only 
a  few  feet  in  width.  It  is  made  up  of  feldspar,  hornblende  and  mica 
imbedded  in  a  groundmass  of  quartz  and  feldspar,  possessing  typical 
microgranitic  structure.  Apparently  this  dike  itself  exerted  little,  if  any, 
influence  on  the  adjoining  country,  and  the  only  geological  interest  at- 
tached to  the  occurrence  consists  in  its  being  closely  allied  to  the  larger 
bodies  of  coarse  granite-porphyry,  from  which  it  is  most  likely  an  offshoot. 
It  is  quite  possible  that  the  quaquaversal  dip  of  the  strata  from  White  Cloud 
Peak,  of  which  mention  has  already  been  made,  may  be  due  to  an  under- 
lying mass  of  intruded  crystalline  rock,  of  which  the  dike  is  the  only 
evidence  upon  the  surface. 

Coinciding  in  direction  with  the  secondary  off-shoots  from  the  main 
dike  occur  narrow  dikes  of  granite-porphyry  penetrating  the  Lone  Moun- 
tain limestone  of  Castle  Mountain.  They  are  exceptionally  fine  grained, 
with  a  characteristic  microgranitic  grouudmass.  In  their  mode  of  occur- 
rence they  resemble  the  dike  near  Bellevue  Peak,  and  doubtless  have  the 


same  common  ongm. 


As  the  geological  and  petrographical  features  of  the  granite-porphyry 
are  discussed  with  some  detail  in  chapter  vu,  devoted  to  the  discussion  of 
the  pre-Tertiary  crystalline  rocks,  it  is  needless  to  enter  more  at  length 
into  the  subject  here.  By  reference  to  the  map  (atlas  sheet  xi)  the 
position  of  the  main  body  of  granite-porphyry  and  its  relations  to  the 
primary  and  secondary  offshoots  from  the  parent  mass  may  be  readily  seen. 

Ridge  west  of  Wood  Cone.— In  many  respects  the  best  locality  to  study  the 
Pogonip  of  the  Eureka  District  is  the  long,  narrow,  monotonous  ridge  which 
stretches  westward  from  Wood  Cone.  Here  the  beds  abut  against  the 
southern  end  of  the  main  granite-porphyry  body,  standing  invariably  at 
high  angles,  in  most  places  nearly  vertical,  but  sometimes  inclined  westerly 
and  again  easterly.  Just  west  of  the  limestone  saddle,  which  separates  the 
two  bodies  of  porphyry,  there  is  a  fault  in  the  limestone  which  brings  up 
the  lower  beds.  There  is  apparently  a  synclinal  fold,  to  the  west  of  which 


THICKNESS  OF  POGONIl'  BEDS.  123 

comes  in  a  sharp  anticline,  beyond  which  the  beds  dip  uniformly  to  the 
west.  At  the  western  end  of  this  ridge  occurs  a  small  knoll  or  hill  of  Eureka 
quartzite,  its  geological  position  being  determined  by  the  Eeceptaculites  fauna 
immediately  underlying  it. 

At  the  eastern  end  of  this  ridge,  just  west  of  Wood  Cone,  a  fauna  was 
obtained  which  indicated  a  horizon  not  far  above  the  base  of  the  Pogonip, 
being  largely  made  up  of  species  found  near  the  summit  of  the  Cambrian, 
associated  with  others  never  as  yet  recognized  below  the  Pogouip.  It  is  a 
fauna  characteristic  of  the  lower  portions  of  the  epoch  and  quite  like  a 
grouping  found  on  the  east  side  of  Hamburg  Ridge.  In  other  words,  they 
may  be  correlated  with  the  transition  beds  just  above  the  Hamburg  shale. 
Many  of  the  species  also  characterize  the  Pogonip  of  White  Pine.  Among 
the  species  identified  were  the  following: 

Lingulepis  mj«ra.  Orthis  hamburgensis. 

Lingula  manticula.  Triplesia  calcifera. 

Leptajna  inelita.  Bathyurus  congeneris. 

Illaenurus  eurekeusis.  Bathyurus  tuberculatus. 

No  accurate  measurements  of  the  Pogonip  along  this  ridge  can  be 
made,  owing  to  the  great  irregularities  of  dip  and  strike,  but  it  is  probable 
that  the  beds  exceed  3,000  feet  in  thickness.  From  the  fauna  obtained 
just  below  the  Eureka  quartzite,  and  that  from  the  base  of  the  limestone 
west  of  Wood  Cone,  it  is  evident  that  the  entire  development  of  Pogonip 
is  represented  in  this  ridge.  This  gives  a  somewhat  greater  development 
for  the  epoch  than  has  been  recognized  east  of  the  Prospect  Ridge,  but,  on 
the  other  hand,  it  does  not  reach  the  very  great  thickness  found  on  Pogonip 
Mountain  at  White  Pine,  estimated  at  5,000  feet. 

REGION    BETWEEN    FISH    CREEK    MOUNTAINS    AND    PROSPECT    RIDGE. 

This  region  possesses  some  distinctive  features  unlike  either  of  the 
mountain  blocks  that  adjoin  it,  yet  at  the  same  time  it  shows  the  influence 
of  the  forces  that  uplifted  Prospect  Ridge  on  the  northeast  and  Fish  Creek 
Mountains  on  the  southwest.  It  is  sharply  denned  from  Prospect  Ridge  in 
geological  structure  by  the  Sierra  fault,  which  brings  tin-  Silurian  up 
against  the  lower  Cambrian  of  Prospect  Ridge.  The  anticlinal  structure 


124  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

of  the  latter  ridge  has  disappeared,  in  place  of  which  there  is  a  complicated 
and  confused  mass  of  mountains  without  any  well  denned  characters.  The 
same  dynamic  forces  that  produced  the  great  longitudinal  faults  extending 
across  the  Eureka  Mountains,  on  both  sides  of  Prospect  Ridge,  may  still 
be  seen  westward  of  the  Sierra  fault  in  a  series  of  north  and  south'  fractures, 
approximately  parallel  with  the  more  powerful  displacements.  Such  lesser 
faults  as  the  Lookout  Mountain,  Pinnacle  Peak,  and  Lamoureux  Canyon 
faults,  are  by  no  means  as  persistent  as  the  Hoosac  and  Pinto,  and  nowhere 
indicate  such  profound  displacements.  The  forces  that  caused  these  dis- 
placements died  out  gradually  to  the  west  of  the  Sierra  fault. 

From  Fish  Creek  Mountains  the  line  of  demarcation  is  by  no  means 
as  easily  denned,  being  unaccompanied  by  great  physical  breaks  of  any 
kind  or  abrupt  changes  in  geological  structure.  The  simplicity  of  the  Fish 
Creek  Mountains  as  they  approach  Prospect  Ridge  gradually  gives  way  to 
a  more  intricate  structure,  the  north  and  south  displacements  being  compli- 
cated by  numerous  minor  cross-fractures  and  faults.  North  of  Castle  Moun- 
tain, the  configuration  of  the  country  gradually  assumes  new  forms,  and 
from  here  to  Prospect  Peak  it  suggests  little  in  common  with  the  ordinary 
type  of  Great  Basin  ranges.  This  intermediate  region  is  the  resultant  of 
varying  forces  not  always  easy  to  define. 

The  Eureka  quartzite  forms  the  surface  rock  over  the  greater  part  of 
this  area,  stretching  in  an  almost  unbroken  line  from  Spring  Valley  to  the 
Sierra  fault,  although  faulting  or  erosion  has  exposed  the  underlying  Pog- 
onip  limestone  in  a  number  of  places.  Overlying  the  Eureka  quartzite 
comes  the  Lone  Mountain,  usually  passing  into  the  Nevada  limestone  of  the 
Devonian,  the  latter  in  the  neighborhood  of  Atrypa  Peak  offering  an 
exposure  several  thousand  feet  in  thickness.  Everywhere  the  Eureka 
quartzite  serves  readily  as  a  datum  point  to  determine  the  position  of  the 
faulted  strata,  and  in  most  instances  the  age  of  the  underlying  beds  may  be 
identified  by  the  Receptaculites  fauna.  Where  the  thickness  of  overlying 
limestone  admits  of  it,  the  Devonian  age  is  shown  by  characteristic 
organic  forms.  By  these  two  groupings  of  fossils  and  the  intermediate 
broad  belt  of  quartzite,  the  stratigraphical  position  of  beds  in  this  highly 
disturbed  region  may  generally  be  determined  without  difficulty. 


ATEYPA  PEAK.  125 

Castle  Mountain  may,  for  sake  of  convenience,  be  taken  as  the  northern 
limit  of  the  Fish  Creek  Mountains.  From  Castle  Mountain  to  Reese  and 
Berry  Canyon  no  beds  come  to  the  surface  other  than  the  quartzites.  Here, 
however,  a  sudden  change  takes  place,  the  canyon  occupying  a  line  of 
southeast  and  northwest  faulting  with  the  quartzite  on  one  side  dipping  at  a 
low  angle  to  the  west,  and  the  Lone  Mountain  limestone  on  the  opposite 
side,  but  without  any  distinct  line  of  bedding.  From  the  head  of  Reese 
and  Berry  Canyon  the  limestone  crosses  over  a  low  saddle  to  the  head  of 
Lamoureux  Canyon,  following  the  latter  ravine  until  it  makes  an  abrupt 
bend  to  the  south.  The  limestone  may  be  traced  eastward  around  the  base 
of  Atrypa  Peak,  thence  westward  again  with  an  irregular  course  as  far  as 
Spring  Valley.  In  this  area  the  underlying  limestone  belongs,  for  the  most 
part,  to  the  Silurian,  but  in  one  or  two  places  the  beds  assigned  to  the 
Devonian  on  lithological  grounds  rest  directly  upon  the  quartzites  abutting 
against  them  almost  at  right  angles.  The  division  between  the  Silurian 
and  Devonian  in  this  region  is  an  arbitrary  one,  but  in  most  instances  the 
passage  from  the  white  saccharoidal  limestone  of  the  former  into  the  strati- 
fied gray  beds  of  the  latter  is  the  same  here  as  elsewhere  in  the  District. 

Atrypa  Peak.— Nowhere  in  this  area  is  there  any  place  which  permits  of  a 
measurement  of  the  Silurian  rocks,  but  the  region  of  Atrypa  Peak,  the  cul- 
minating point,  affords  excellent  sections  across  the  Nevada  limestone,  the 
beds  presenting  nearly  uniform  dips  and  strikes.  This  imposing  mountain 
is  formed  almost  wholly  of  Devonian  limestone,  the  name  of  the  peak 
being  derived  from  the  abundance  of  Atrypa  reticularis  found  on  its  slopes. 
Two  sections  for  comparative  purposes  were  made  :  one,  directly  across  the 
strata  on  the  southeast  slope  of  the  peak,  the  other  on  the  high  ridge 
extending  westward  lying  between  the  peak  and  the  head  of  Lamoureux 
Canyon.  The  latter  section  will  be  found  on  page  67. 

Where  the  sections  include  the  same  geological  horizons  they  agree 
closely  in  details,  but  the  one  taken  across  the  slope  of  the  peak  gives  a 
much  greater  thickness  of  Silurian  rocks,  whereas  the  ridge  section  ex- 
tends higher  up  into  Devonian  strata.  The  fossiliferous  shaly  belt  (No.  5), 
in  the  section  east  of  Lamoureux  Canyon,  is  easily  traceable  across  the 
ravine  to  Atrypa  Peak  and  may  be  taken  as  a  base  for  comparing  the 


126  GEOLOGY  OP  THE  ETJEEKA  DISTRICT. 

two  sections.  In  the  ridge  section  there  are  1,300  feet  of  strata  below  this 
shale  belt  before  reaching  the  quartzite,  and  about  3,000  feet  above  the 
shale.  The  Atrypa  Peak  section  gives  2,000  feet  from  the  shale  to  the 
quartzite  at  the  base,  and  nearly  the  same  thickness  from  the  shale  upward. 
This  shale  carries  an  abundance  of  characteristic  species  and,  although  a 
larger  number  were  obtained  on  the  slope  of  Atrypa  Peak,  there  is  no 
question  that  the  fauna  is  identical  in  both. 

At  the  head  of  Lamoureux  Canyon  there  is  a  ridge  of  limestone, 
striking  northwest  and  southeast,  which  rests  unconformably  against  the 
quartzite.  Not  far  above  the  quartzite  a  small  collection  of  typical  fossils 
was  made,  amply  sufficient  to  prove  that  the  beds  belong  to  the  Devonian. 
On  the  summit  of  the  high  peak  east  of  Jones  Canyon  is  another  excellent 
locality  for  the  collection  of  Devonian  species,  but  no  specific  forms  were 
found  here  not  recognized  elsewhere.  Owing  to  local  faulting,  the  exact 
position  of  these  latter  beds  could  not  be  determined  other  than  that  they 
belonged  to  the  lower  Nevada  limestone.  They  are  well  bedded,  strike 
across  the  ridge  and  dip  westerly. 

Jones  Canyon  lies  wholly  in  the  Devonian  limestone  and  offers  some 
good  exposures  of  rock,  but  no  continuous  section  at  all  comparable  to  those 
described  in  the  region  of  Atrypa  Peak. 

white  Mountain.— The  country  between  Atrypa  Peak,  and  the  Prospect 
Peak  fault  culminates  in  White  Mountain  (9,941),  the  highest  point  west 
of  Prospect  Ridge,  with  which  it  is  connected  on  the  northeast  by  a  high 
ridge  of  quartzite.  From  Spring  Valley  a  fairly  uniform  slope  of  1,500 
feet  extends  to  the  summit  of  White  Mountain,  made  up  wholly  of  Pogonip 
limestone,  which  stretches  eastward  and  falls  away  gradually  for  about  800 
feet  to  a  high  saddle  in  the  range,  beyond  which  it  descends  in  a  narrow 
belt  for  another  300  feet  to  Mountain  Valley.  Here  it  is  cut  off  by  a  fault 
bringing  up  a  narrow  strip  of  Nevada  limestone  lying  between  the  Pogonip 
on  the  one  side  and  the  Eureka  quartzite  on  the  other.  It  is  possible  that 
this  fault  may  be  only  an  extension  northward  of  the  Pinnacle  Peak  fault. 
In  the  neighborhood  of  the  saddle  the  quartzite  encroaches  on  the  lime- 
stone. The  structure  of  the  mountain  is  difficult  to  make  out,  but  the 
limestone  is  everywhere  surrounded  by  the  quartzite,  long  belts  of  the 


WHITE  MOUNTAIN  KEGION.  127 

latter  rock  stretching  down  on  both  the  north  and  south  sides  of  the  moun- 
tain to  Spring  Valley.  Patches  of  quartzite  resting  upon  the  limestone  on 
the  summit  give  stratigraphical  evidence  of  the  age  of  the  beds.  It  is  proba- 
ble that  the  quartzite  passed  over  the  top  of  the  limestone,  east  of  the 
mountain,  and  that  the  patches  of  the  former,  found  near  the  summit,  are 
mere  relics  of  erosion.  As  regards  stratigraphic  position  of  beds,  we  have 
here  conditions  nearly  identical  to  those  in  the  Fish  Creek  Mountains. 
Characteristic  Pogonip  fossils,  sufficient  to  determine  the  position  of  the 
beds,  have  been  secured  from  a  number  of  localities,  proving  the  age  of  the 
limestone,  while  the  beds  forming  the  summit  have  furnished  a  typical 
fauna  of  the  upper  portions  of  this  horizon.  About  800  feet  below  the  top 
of  the  mountain  and  not  far  from  the  same  distance  below  the  quartzite 
bodies  an  interesting  grouping  of  fossils  occurs,  and  immediately  beneath 
the  quartzite  on  the  summit  the  Eeceptaculites  beds  are  well  shown.  The 
student  of  structural  geology  in  this  region  owes  much  to  the  genus  Recep- 
taculites,  which  is  very  abundant  within  a  restricted  vertical  range.  A  list 
of  the  principal  groupings  of  fossils  collected  on  White  Mountain  will  be 
found  on  page  52. 

South  of  White  Mountain,  and  separated  from  it  by  a  belt  of  Eureka 
quartzite  not  over  1,000  feet  in  width,  an  irregular  shaped  body  of  lime- 
stone is  exposed  from  beneath  the  quartzite.  If  any  evidence  of  its  age  is 
needed  beyond  its  stratigraphical  position,  it  will  be  found  in  the  typical 
Pogonip  fossils  which  occur  scattered  throughout  the  beds  which,  like  the 
corresponding  beds  on  the  east  slope  of  White  Mountain,  possess  a  south- 
east dip  and  a  northeast  and  southwest  strike.  This  limestone,  like  the 
main  body,  is  nearly  everywhere  encircled  by  the  quartzite,  the  only  ex- 
ception being  on  the  south  side,  where  it  abuts  against  the  Nevada  lime- 
stone, which  forms  a  part  of  the  east  ridge  of  Atrypa  Peak.  The  two 
limestone  bodies  are  unconformable,  of  different  lithological  character,  and 
dip  in  opposite  direction. 

North  of  White  Mountain  the  Eureka  quartzite  terminates  abruptly 
against  the  Prospect  Peak  fault,  the  Cambrian  and  Silurian  quartzites 
being  placed  in  juxtaposition.  These  quartzites  resemble  each  other 
closely  in  their  upper  strata,  being  simply  indurated  sandstones,  and  it  is 


128  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

only  after  long  study  of  them  that  they  can  be  readily  distinguished; 
along  the  line  of  contact  it  is  by  no  means  easy  to  separate  them.  Evi- 
dences of  geological  position  come  in,  however,  and  the  limestone,  both 
above  and  below  the  Eureka  horizon,  usually  determines  the  age  of 
the  beds.  As  the  country  is  much  broken  up  by  profound  faults,  and 
the  Eureka  quartzite  is  not  over  500  feet  in  thickness,  either  the  Pogonip 
below  or  Lone  Mountain  horizon  above,  frequently  both,  are  apt  to  come 
to  the  surface  near  the  exposures  of  the  Silurian  quartzite.  Wherever  the 
Cambrian  quartzite  is  found  it  is  overlain  by  Cambrian  limestone. 

On  the  summit  of  the  ridge  along  the  line  of  the  Prospect  Peak  fault 
occurs  a  small  patch  of  highly  altered  limestone,  without  any  structural 
indications  of  its  relationship  to  either  of  the  quartzite  bodies.  Its  position  is 
difficult  to  explain  satisfactorily,  but  it  has  been  referred  to  the  Pogonip, 
since  it  more  closely  resembles  the  limestone  of  White  Mountain  than  that 
of  Prospect  Ridge. 

From  Prospect  Peak  southward  the  Eureka  quartzite  forms  the  west 
side  of  Prospect  Ridge,  following  the  line  of  the  Sierra  fault.  The  ridge 
falls  away  steadily  to  the  south  for  1^  miles,  with  a  descent  of  over 
1,500  feet  to  Sierra  Valley.  A  series  of  minor  longitudinal  faults  pre- 
sents a  much  more  abrupt  slope  on  the  west  side  and  prevents  the 
underlying  formations  from  coming  to  the  surface,  notwithstanding  that  a 
narrow  ravine  is  eroded  in  the  quartzites  for  nearly  700  feet  in  depth.  Not 
till  descending  the  slope  for  nearly  1,000  feet  do  the  Pogonip  beds  come 
to  the  surface,  and  then  only  a  small  patch  of  this  underlying  rock  is 
exposed.  This  interesting  body  of  limestone  crops  out  to  the  northeast  of 
Lookout  Mountain,  where  it  presents  an  obscure  exposure  of  slight  area 
and  thickness.  The  fauna  obtained  here  is  strikingly  Pogonip  in  aspect, 
and  resembles  the  fauna  found  on  the  face  of  White  Mountain  for  500  to 
1,000  feet  below  the  summit.  Associated  with  other  more  common  forms 
are  Raphistoma  nasoni,  Maclurea  annulata,  and  Leperditia  livia,  all  recog- 
nized as  belonging  to  the  Pogonip  of  White  Pine.  The  interest  in  this 
identification  lies  in  the  fact  that  only  a  few  hundred  feet  to  the  southward 
the  Cambrian  limestone  comes  to  the  surface  in  Sierra  Valley,  while  just  to 


FAULTED  LIMESTONE  BLOCKS.  129 

the  westward  the  Devonian  limestone  is  exposed  in  Mountain  Valley,  the 
three  horizons  being  determined  by  characteristic  species. 

Lookout  Mountain.— This  isolated  mountain  stands  out  prominently  from 
the  surrounding  country,  cut  off  on  three  sides  by  faults.  On  the  east  runs 
the  Lookout  fault,  and  along  the  west  base  the  persistent  and  profound 
Pinnacle  Peak  fault  brings  up  the  Nevada  limestone  against  the  Eureka 
quartzite.  The  mountain  is  wholly  made  up  of  quartzite,  inclined  eastward 
at  low  angles,  the  beds  of  which  are  for  the  most  part  darker  in  color  and 
more  ferruginous  than  those  of  the  same  horizon  found  elsewhere.  At  the 
east  base  of  the  mountain  occurs  a  small  patch  of  limestone,  in  part 
obscured  by  surface  accumulations  of  Sierra  Valley  and  in  part  by 
andesitic  lavas.  As  this  limestone  lies  on  the  east  side  of  the  Lookout 

• 

fault  its  age  can  be  determined  only  by  its  fauna,  but  fortunately  this  is 
sufficiently  typical  to  admit  of  its  reference  to  the  Cambrian. 

Northward  of  this  last  exposure  and  separated  from  it  by  only  300  feet 
of  acidic  lavas,  occurs  a  larger  body  of  limestone,  which  forms  a  narrow 
ridge,  cut  by  the  stream  bed  which  comes  down  along  the  north  side  of 
Lookout  Mountain.  The  ravine  affords  a  fair  exposure  of  the  beds.  This 
second  body  of  limestone  presents  no  structural  evidence  of  its  position, 
the  fauna  alone  determining  its  age,  but  fortunately  it  yielded  a  small  num- 
ber of  fossils.  These  two  groupings  are  not  quite  identical,  but  the  beds 
from  which  they  were  obtained  can  not  be  wide  apart.  The  outcrop  east  of 
Lone  Mountain  indicates  clearly  the  horizon  of  the  Hamburg  limestone, 
carrying  certain  species  which  extend  downward  into  the  Prospect  Moun- 
tain beds,  mingled  with  others  occurring  as  high  as  the  middle  portion  of 
the  Pogonip.  The  larger  exposure  at  the  northeast  base  of  the  mountain 
has  been  assigned  to  the  Prospect  Mountain  limestone,  without  any  decided 
evidence  as  to  the  correctness  of  the  reference  otherwise  than  that  it  belongs 
to  the  Cambrian. 

Pinnacle  Peak.— This  summit  lies  about  one  and  one-quarter  miles  due 
south  of  Lookout  Mountain  and  presents  much  the  same  general  features 
in  the  character  of  the  beds  and  mode  of  occurrence,  the  two  mountains 
being  connected  by  a  continuous  mass  of  quartzite.  The  beds  strike 
invariably  north  and  south  and  incline  eastward  at  angles  si-Mom 
MON  xx 9 


130  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

exceeding  20°,  forming  the  entire  slope  as  far  as  the  Lookout  fault.  There 
is  little  doubt  that  this  quartzite  is  correctly  referred  to  the  Silurian, 
although  no  direct  evidence  exists.  Nearly  everywhere  else  the  Eureka 
quartzite  may  be  determined  upon  structural  grounds  alone,  but  here  the 
entire  body  from  Lookout  Mountain  to  Pinnacle  Peak  has  been  uplifted 
between  two  longitudinal  faults,  with  limestones  of  different  age  brought 
to  the  surface  on  opposite  sides  of  the  displacements  and  lying  unconform- 
ably  against  the  quartzite.  In  contrast  with  the  quartzite  on  the  west  side 
of  the  Lookout  fault,  limestones  form  the  east  wall  stretching  southward 
until  beds  on  both  sides  of  the  fault  are  buried  beneath  volcanic  lavas. 
This  body  of  limestone  extends  eastward  until  cut  off  by  the  fault,  bring- 
ing up  the  basal  members  of  the  Cambrian  limestone  of  Prospect  Ridge. 
Between  these  two  faults  the  beds  are  broken  by  irregular  outbursts  of 
andesites  and  in  places  have  undergone  considerable  alteration,  due  to  sol- 
fataric  action,  the  beds  being  frequently  intersected  by  calcite  and  quartz  in 
naiTOW  seams  and  veins.  So  much  disturbed  are  the  beds  that  structural 
features  are  of  little  value,  although  it  may  be  well  to  add  that  the  general 
dip  is  eastward.  These  limestones  have  been  referred  to  the  Pogonip, 
although  evidence  of  their  position  is  not  in  all  respects  satisfactory. 
Obscure  fragments  of  fossils  may  be  obtained  in  a  number  of  places,  but 
only  in  one  was  anything  like  a  grouping  of  forms  observed.  This  fauna 
was  collected  on  the  west  side  of  Sierra  Canyon,  nearly  due  south  from 
Surprise  Peak  and  just  west  of  the  Prospect  Mountain  limestone,  in  dis- 
tinctly bedded  strata  inclined  at  an  angle  of  about  40°  eastward.  All  the 
species  obtained  have  been  found  in  the  Pogonip  limestones  elsewhere,  but 
singularly  enough  they  are  all  known  in  the  Hamburg  limestone,  every 
species  having  a  wide  vertical  range.  They  probably  represent  beds  not 
far  from  the  base  of  the  Pogouip  and  possibly  should  be  referred  to  the 
same  horizon  as  the  beds  east  of  Lookout  Mountain,  although  at  the  latter 
locality  the  fauna  distinctly  indicates  the  Hamburg  period.  This  refer- 
ence to  the  Pogonip,  however,  is  justified  by  the  occurrence  of  undoubted 
Silurian  beds  underlying  Surprise  Peak;  a  further  search  would  certainly 
determine  the  question. 


SUKPK1SE  PEAK.  131 

Surprise  Peak.-No  mountain  in  this  part  of  the  district  affords  a  more 
commanding  view  than  Surprise  Peak.  It  is  situated  between  the  Sierra 
fault  on  the  east  side  and  Sierra  Valley  on  the  west.  It  is  capped  by 
Eureka  quartzite,  which  is  underlain  by  the  Pogonip,  the  limestone  being 
distinctly  seen  to  pass  beneath  the  quartzite.  On  the  north  side  of  the 
peak,  and  on  the  opposite  side  of  the  fault,  in  beds  unconformable  with 
the  Prospect  Mountain  limestone,  was  found  a  small  but  characteristic 
Pogonip  fauna.  Its  occurrence  here  is  so  important  that  it  is  given  in  full, 
as  follows: 

Keceptaculites  mainmillaris.  Kaphistoma  nasoni. 

Cystidean  plates.  Pleurotomaria? 

Orthis  perveta.  Leperditia  bivia. 
Orthis  tricenaria. 

Sierra  Valley,  along  the  west  base  of  Surprise  Peak,  has  been  the 
center  for  the  eruption  of  considerable  masses  of  andesitic  pearlites  and 
hornblende  audesites,  which,  in  the  form  of  small  irregular  knolls  and  dikes, 
have  penetrated  the  limestone  on  the  south  side  of  the  peak.  Associated 
with  these  dikes  are  others  of  rhyolite,  while  still  farther  southward,  where 
the  sedimentary  rocks  pass  beneath  the  valley,  occur  large  accumulations 
of  pearlites,  pumices,  and  tuffs.  Details  in  regard  to  these  igneous  rocks 
will  be  found  on  page  234  et  seq. 

Grays  Canyon.— The  Pinnacle  Peak  fault  lies  on  the  west  side  of  the 
peak  of  the  same  name,  at  the  southern  end  of  the  mountains.  The 
line  of  the  fault  is  obscured  by  broad  lava  flows,  but  where  these  give  out 
it  is  easily  traceable  northward  nearly  to  Prospect  Peak  with  the  Eureka 
quartzite  on  one  side  and  the  Nevada  limestone  on  the  other. 

West  of  the  Pinnacle  Peak  fault  the  Nevada  limestone  extends  from 
Mountain  Valley  southward  till  the  sedimentary  beds  pass  beneath  Fish 
Creek  Valley.  Through  these  limestones  Grays  Canyon  cuts  a  narrow 
ravine,  which  offers  a  few  good  exposures,  but  nowhere  exhibits  a  continu- 
ous sectioa  across  any  great  thickness  of  beds.  Only  the  lower  portions  of 
the  Nevada  limestone  are  exposed,  and  over  the  greater  part  of  this  area 
bedding  planes  are  wanting.  The  best  locality  observed  tor  the  collection 
of  fossils  was  found  on  the  low,  flat-topped  ridge  west  of  (ir;iys  Canyon 


132  (1KOUH1Y  OF  THE  EUltEKA  DISTRICT. 

and  southwest  of  Pinnacle  Peak,  the  beds  dipping  to  the  southeast  at  a  low 
angle  and  striking  northeast  and  southwest.  These  beds  yielded  the  fol- 
lowing forms: 

Thecia  ramosa.  Dystactella  iusularis. 

Aulopora  serpens.  Conocardium  nevadeusis. 

Chonetes  deflecta.  Loxonema  subattenuata. 

Spirifera  piiioneusis.  Bellerophon  perplexa. 

Atrypa  retieularis.  Tentaculites  scalariformis. 
Rhynchonella  occidens. 

Nearly  all  these  species  occur  in  the  shale  belts  of  Atrypa  Peak,  Brush 
Peak,  and  Combs  Mountain,  the  exceptions  being  the  three  species,  Thecia 
ramosa,  Aulopora  serpens,  and  Dystactella  insularis,  which  are,  however, 
characteristic  of  the  upper  Helderberg  in  New  York  and  Ohio;  Thecia 
ramosa  and  Dystactella  insidaris  have  only  as  yet  been  found  at  this  one 
locality  at  Eureka.  A  smaller  but  somewhat  similar  grouping  of  fossils 
occurs  in  the  limestone  just  west  of  Lookout  Mountain,  where  they  are 
associated  with  Strophodonta  canace,  a  species  found  by  the  writer  in  the 
limestone  at  Treasure  Hill,  White  Pine. 

On  the  west  slope  of  Pinnacle  Peak  the  beds  dip  toward  the  fault  at 
an  angle  of  10°,  reaching  to  within  150  feet  of  the  summit  and  lying  un- 
conformably  against  the  Eureka  quartzite  of  the  peak.  Following  the  line 
of  the  fault  the  beds  trend  off  to  the  southeast,  the  quartzite  belt  gradually 
narrowing  until  lost  beneath  the  pumices,  the  Nevada  limestone,  on  the 
other  hand,  continuing  southward  in  a  low  ridge  bounded  on  the  east  and 
west  sides  by  igneous  rocks.  The  beds  exhibit  much  the  same  habit  as 
those  to  the  northward,  usually  light  in  color  and  highly  siliceous,  but  show- 
ing more  distinct  lines  of  bedding.  By  reference  to  the  map  (atlas  sheet  xi) 
the  structure  will  be  seen  indicated  by  strikes  and  dips.  South  Hill,  the 
most  prominent  point  on  this  southern  extension,  has  a  marked  anticlinal 
fold,  the  axis  of  the  fold  striking  N.  40°  to  45°  east,  with  a  dip  of  15°. 
The  brownish  gray  limestones  are  distinctly  bedded  and  probably  belong  to 
a  somewhat  higher  horizon  than  any  of  those  exposed  in  Grays  Canyon. 
South  of  the  road,  which  traverses  the  ridge  near  its  southern  extremity,  a 
well  defined  but  gentle  synclinal  fold  may  be  seen  crossing  the  ridge 


GRAYS  PEAK.  133 

obliquely,  with  approximately  the  same  strike  as  the  strata  on  South  Hill. 
In  this  southern  extension  the  only  fossils  obtained  were  ChaeMes  and  as- 
sociated corals  so  abundant  in  the  Lower  Nevada  limestone. 

Grays  Peak.— This  name  has  been  given  to  the  flat  topped  summit  which 
forms  the  eastern  limit  of  the  broad  quartzite  plateau.  It  offers  a  command- 
ing1 view,  as  the  country  falls  off  rapidly  to  the  south  and  east.  On  the 
summit  the  beds  lie  nearly  horizontal,  but  break  away  abruptly  and  dip 
off  iu  every  direction  accompanied  by  mural-like  escarpments  produced  by 
a  series  of  small  parallel  faults  lying  wholly  within  the  quartzite.  On  the 
eastern  side  the  slope  descends  for  nearly  1,000  feet,  with  an  average  dip 
of  20°,  the  angle  of  the  slope  and  the  inclination  of  the  beds  coinciding 
within  1°  or  2°.  South  and  east  the  quartzites  are  overlain  by  the  Nevada 
limestones  which  dip  away  from  the  peak  with  varying  angles.  On  the  east 
side  the  line  of  contact  between  the  two  formations  is  strongly  marked  by 
a  deeply  eroded  ravine  draining  into  Grays  Canyon.  While  these  lime- 
stones have  been  referred  to  the  Nevada  period,  it  is  by  no  means  definitely 
ascertained  that  beds  which  in  other  places  have  been  assigned  to  the  Lone 
Mountain  series  may  not  here,  in  some  instances,  rest  upon  the  quartzite. 
In  many  instances  there  is  an  entire  absence  of  bedding,  and  in  others  the 
strata  rest  unconformably  upon  the  quartzite.  Apparently  the  underlying 
limestones  belong  to  the  transition  series  between  well  recognized  Silurian 
and  Devonian,  but  pass  rapidly  into  limestone  which  has  everywhere  else 
in  the  district  been  assigned  to  the  Nevada  epoch.  These  limestones  stretch 
away  to  the  south  in  insignificant  monotonous  hills  and  ridges  of  lower 
Devonian  age  and  have  as  yet  yielded  only  a  few  obscure  corals  of  wide 
vertical  range.  North  of  Grays  Peak  on  the  plateau  where  the  beds  lie 
either  horizontally  or  at  low  angles,  there  are  several  patches  of  limestone 
still  left  in  place  as  remnants  of  erosion.  These  exposures  resemble  the 
beds  of  the  Lone  Mountain  series  and  serve  to  show  by  their  geological 
position  that  the  quartzites  on  the  ridge  belong  to  the  upper  members  of 
the  Eureka  epoch.  To  the  westward  of  these  Silurian  limestone  patches 
the  quartzites  break  down  in  abrupt  walls  and  cliffs  toward  Lamoureux 
Canyon  much  in  the  same  way  as  seen  on  the  east  side  of  Grays  Peak. 
Along  Lamoureux  Canyon,  however,  the  wall  is  most  persistent,  continuing 


134  GEOLOGY  OP  THE  EUREKA  DISTRICT. 

northward  nearly  to  Atrypa  Peak,  and  is  an  excellent  locality  for  studying 
the  Eureka  quartzite.  A  longitudinal  fault  line  follows  up  Lamoureux 
Canyon,  but  the  amount  of  movement  is  by  no  means  as  great  as  along  the 
Sierra  and  Lookout  faults ;  the  orographic  movements  apparently  dis- 
playing less  and  less  force  to  the  westward  of  Prospect  Ridge.  Passing  up 
to  the  head  of  Lamoureux  Canyon,  there  is  an  interesting  occurrence  of  an 
exposure  of  the  underlying  limestones  brought  up  by  faulting.  Here  the 
Pogonip  beds  are  surrounded  on  all  sides  unconformably  by  the  quartzite. 
The  hill  in  the  middle  of  the  canyon  formed  of  these  limestones  is  capped 
by  about  100  feet  of  quartzite  resting  conformably  upon  the  underlying 
beds.  A  careful  search  in  this  locality  reveals  the  Receptaculites  fauna, 
associated  with  Orthis  and  Maclurea,  immediately  beneath  the  quartzite. 

Between  Lamoureux  Canyon  and  Castle  Mountain  the  country  presents 
the  appearance  of  a  shallow  trough  or  basin  with  a  northwest  and  southeast 
trend.  This  basin  is  for  the  most  part  filled  with  Nevada  limestone,  between 
which  and  the  Eureka  quartzite  the  Lone  Mountain  beds  generally  come 
to  the  surface,  forming  a  narrow  belt  around  the  edge  of  the  basin  and  in 
places  extending  up  on  to  the  top  of  the  quartzite  rim.  Over  this  area  the 
beds  dip  east  and  southeast  except  immediately  next  the  quartzite  of  Lam- 
oureux Canyon,  where,  conforming  with  it,  they  show  a  westerly  dip.  But 
few  fossils  have  been  recognized  in  this  area  other  than  an  occasional  Atrypa 
reticularis  and  corals  characteristic  of  the  Devonian,  but  without  indicating 
any  special  horizon. 

MAHOGANY   HILLS. 

Spring  Valley  extends  the  entire  length  of  the  Eureka  Mountains  and 
sharply  distinguished  Prospect  Ridge  and  the  Fish  Creek  Mountains  from 
Mahogany  Hills,  all  that  region  lying  on  the  west  side  of  this  valley  being 
included  within  the  Mahogany  Hills.  Strictly  speaking,  it  is  not  one 
continuous  valley,  but  rather  two  valleys,  with  a  low  dividing  grassy 
ridge  between  them,  the  water  draining  both  to  the  north  and  to  the 
south.  From  the  broad  plain  of  Diamond  Valley,  Spring  Valley,  only  a 
few  hundred  yards  in  width,  rises  gradually  for  1,200  feet  to  the  divide, 
following  the  course  of  a  remarkable  fault,  which  brings  both  the  Lone 


COMBS  PEAK.  135 

Mountain  and  the  Nevada  limestones  in  juxtaposition  with  the  Prospect 
Mountain  quartzite,  recent  accumulations,  however,  obscuring  the  precise 
line  of  the  displacement.  The  water-shed  lies  nearly  opposite  Prospect 
Peak.  Southward  from  this  dividing  ridge  the  valley  becomes  a  more  im- 
portant physical  feature,  in  places  opening  out  to  more  than  a  mile  in  width, 
finally  draining  into  Antelope  Valley  southwest  of  the  mountains.  The 
southem  end  of  the  valley  is  arid  and  covered  with  sage-brush,  closely 
resembling  the  broader  longitudinal  valleys  of  the  Great  Basin. 

Mahogany  Hills  occupy  by  far  the  largest  area  of  any  mountain  block 
in  the  Eureka  District,  measuring  12  miles  in  length  by  8  miles  in  width. 
Nevada  limestones  constitute  by  far  the  greater  part  of  this  orographic 
block,  four  epochs  of  the  geological  section — Eureka  quartzite,  Lone 
Mountain  limestone,  Nevada  limestone,  and  Diamond  Peak  quartzite — are 
all  represented  and  their  structural  relations  well  shown.  In  presenting 
some  of  the  more  important  details  of  the  region,  it  will  be  well  to  begin  at 
the  southern  end,  where  both  in  geological  and  topographical  structure 
Mahogany  Hills  are  closely  connected  with  the  Fish  Creek  Mountains 
through  Wood  Cone  and  the  granite-porphyry  region. 

Combs  Peak.  On  the  north  side  of  Wood  Cone,  resting  uncomformably 
upon  the  Eureka  quartzite,  lies  a  body  of  bluish  black  and  dark  gray  lime- 
stones dipping  beneath  the  limestones  of  Combs  Peak.  These  dark  lime- 
stones everywhere  form  the  southern  slopes  of  the  Peak,  and  westward  of 
the  quartzite  rest  directly  upon  the  granite-porphyry  body.  The  hillsides 
are  scored  by  frequent  ravines  and  water-courses  showing  the  inclination  of 
the  strata  northward  into  the  mountain,  but  lines  of  stratification  are 
exceedingly  rare,  nowhere  affording,  for  any  considerable  distance,  con- 
tinuous dips  and  strikes.  The  best  locality  for  observing  these  beds  was 
found  just  north  of  Wood  Cone,  on  the  end  of  the  long  spur  coming 
down  from  Combs  Peak.  From  their  dark  steel-gray  color  and  their 
uniformly  fine  grained  appearance,  it  is  easy  to  see  that  they  differ  essen- 
tially from  the  characteristic  Lone  Mountain  beds  observed  elsewhere.  This 
is  all  the  more  noticeable,  as  they  are  found  to  pass  into  beds  possessing 
the  peculiar  habit  of  the  latter  horizon.  This  striking  contrast  in  the  lime- 
stones led  to  a  diligent  search  for  paleontological  evidence  of  their  geologi- 


136  GEOLOGY  OF  THE  EUKEKA  DISTRICT. 

cal  position,  a  search  which  was  rewarded  by  finding  a  limited  and  imper- 
fect fauna,  characteristic  of  the  Trenton  period.  The  finding  of  this  group- 
ing of  fossils  is  important,  as  it  carries  the  comformable  Silurian  limestones 
overlying  the  Eureka  quartzite  down  into  beds  generally  regarded  as  lower 
Silurian,  whereas,  elsewhere  in  the  district  there  is  no  paleontological  evi- 
dence of  strata  older  than  the  Niagara  or  Hall/site*  beds  above  the  quartzite. 
Some  description  of  this  fauna  will  be  found  on  page  59. 

The  dark  limestones  which  have  been  referred  to  the  Trenton  at  this 
point  measure,  according  to  the  best  estimates  that  can  be  made,  about  300 
feet;  that  is  to  say,  this  is  approximately  the  thickness  from  the  Eureka 
quartzite  on  Wood  Cone  to  the  strata  having  the  characteristics  of  the 
horizon  found  elsewhere  and  regarded  as  of  Lone  Mountain  age.  These 
dark  limestones  extend  northward  to  the  low  saddle  over  which  the  wagon 
road  passes,  beyond  which  the  light  colored,  pearly  limestones  come  in. 
Westward  and  northward  of  the  granite-porphyry  a  second  locality  was 
found  yielding  a  similar  fauna,  proving  the  extension  of  the  horizon  in 
that  direction.  Here  the  Trenton  beds,  or  those  assigned  to  that  epoch 
upon  lithological  grounds,  appear  somewhat  thicker  than  those  obtained  near 
the  first  mentioned  locality.  Passing  up  the  slope  of  the  peak  over  the  Lone 
Mountain  beds,  north  of  Wood  Cone,  the  strata  generally  referred  to  the 
Nevada  limestone  make  their  appearance  at  the  base  of  the  first  abrupt 
slope  of  the  long  spur  from  Combs  Peak,  and  from  here  to  the  top  of  the 
prominent  hill  south  of  the  peak  the  ridge  offers  an  excellent  section  across 
the  limestones.  The  beds  strike  across  the  ridge  and  dip  toward  the  peak, 
with  varying  angles.  A  number  of  the  observed  strikes  and  dips  will 
be  found  recorded  on  atlas  sheet  ix.  On  the  top  of  the  hill  a  few  fossils 
may  be  found,  indicating  that  the  beds  at  the  top  of  the  northerly  dipping 
rocks  still  belong  to  the  Lower  Nevada  limestone.  Between  this  hill  and 
the  summit  of  Combs  Peak  occurs  a  sharp  syncline,  the  axis  of  the  fold 
lying  in  the  saddle  at  the  base  of  the  steep  slope  of  the  peak.  The  lime- 
stones on  both  summits  strike  about  N.  55°  west;  those  011  the  peak  dipping 
25°  southwesterly,  and  those  on  the  spur  35°  northeasterly.  The  amphi- 
theater of  Combs  Canyon  has  been  eroded  out  of  the  beds  lying  within 
the  synclinal  fold. 


RHYOLITE  OP  MAHOGANY  HILLS.  137 

OH  the  west  spur  of  Combs  Peak,  in  beds  dipping  to  the  northeast, 
occurs  a  belt  of  calcareous  shales  about  150  feet  in  width,  carrying  a  rich 
and  varied  fauna  quite  similar  to  the  fossil-bearing  shale  belts  of  Atrypa 
and  Brush  peaks  and  with  a  nearly  identical  fauna.  On  page  7<>  will  be 
found  a  list  of  the  Combs  Peak  fauna,  together  with  those  of  the  other 
peaks,  showing  the  strong  parallelism  in  the  life  from  the  three  localities. 
The  precise  locality  from  which  this  fauna  was  obtained  is  designated  on 
the  map.  All  the  beds  on  the  north  slope  of  Combs  Peak  belong  to  the 
east  side  of  the  synclinal  fold,  dipping  into  the  mountain  and  passing 
beneath  the  beds  which  form  .the  summit. 

Browns  Canyon,  at  the  base  of  the  mountain,  lies  in  the  axis  of  an 
anticlinal  fold,  the  beds  on  the  north  side  dipping  to  the  northeast  at  angles 
seldom  exceeding  20°.  At  the  head  of  this  canyon,  along  the  axis  of  the 
fold,  occurs  a  body  of  compact  rhyolite,  which  has  for  the  most  part  been 
extravasated  on  the  south  side  of  a  local  line  of  faulting.  It  forms  a  hill 
about  250  feet  in  height,  whose  outlines  are  sharply  denned  by  drainage 
channels  which  almost  completely  surround  it  on  all  sides.  The  slopes  of 
the  hill  are  strewn  with  fissile,  sherdy  fragments  of  rock  characteristic 
of  the  entire  mass.  The  rhyolite  has  a  microcrystalline  groundmass,  with 
but  few  microscopic  crystals  of  gray  quartz,  brilliant  biotite  flakes,  and 
occasional  dull  orthoclases.  In  the  middle  of  this  rhyolite  is  an  irregular 
exposure  of  Nevada  limestone  about  100  feet  in  thickness,  indicating  that 
the  greater  part  of  the  lava  is  only  a  thin  flow  over  underlying  limestones. 
It  is  the  single  instance  of  a  rhyolite  exposure  observed  in  Mahogany  Hills 
east  of  Yahoo  Canyon. 

Temple  Peak.— From  this  rhyolite  body  the  limestone  hills  rise  gradually 
to  the  northeast  in  gentle,  flat  topped  spurs,  culminating  in  Temple  Peak 
(8,398  feet),  the  highest  point  between  Browns  and  Denio  canyons. 
Across  this  limestone  body,  from  Browns  Canyon  to  Dry  Lake,  the  strata 
dip  persistently  to  the  northeast,  with  a  northwest  and  southeast  strike. 
The  limestones  at  the  summit  lie  inclined  at  angles  seldom  exceeding  5°, 
but  are  distinctly  bedded,  and  in  physical  habit  and  sequence  of  strata 
resemble  those  about  midway  in  the  Nevada  limestone  epoch.  The  same 


138  GEOLOGY  OF  THE  EUEEKA  DISTRICT. 

limestones   cross    Denio   Canyon  and  continue  northward  to   Bnrlingame 
Canyon,  invariably  dipping  slightly  to  the  northeast. 

About  150  feet  above  the  bottom  of  Browns  Canyon,  in  beds  near  the 
base  of  the  Nevada  limestone,  a  small  number  of  fossils  were  procured, 
most  of  them  like  Atrypa  reticularis,  common  forms  having  a  wide  vertical 
range.  Associated  with  them  was  the  coral  Acervulariapentagona.  This  was 
found  also  by  the  writer  in  the  Nevada  limestone  of  Treasure  Hill,1  White 
Pine,  the  only  other  locality  where  it  has  been  observed  in  the  Great  Basin. 

Table  Mountain.— South  of  Browns  Canyon  the  beds  of  Combs  Peak  con- 
tinuing westward  gradually  curve  around  until  the  limestones  of  Table 
Mountain  strike  north  and  south  and  lie  nearly  horizontal,  but  with  a  slight 
dip  to  the  east.  Table  Mountain  is  made  up  of  dark  massive  beds,  the 
upper  strata  occupying  about  the  same  geological  position  as  the  summit  of 
Temple  Peak.  From  Table  Mountain  westward  to  Antelope  Valley,  the 
long  spurs  afford  a  fair  opportunity  to  study  the  beds  of  the  lower  and 
middle  portions  of  the  Nevada  epoch,  which  is  here  represented  by  2,500 
to  3,000  feet  of  limestones. 

Devon  Peak.— The  culminating  point  of  the  northwest  part  of  Mahog 
any  Hills  is  known  as  Devon  Peak  (8,537  feet),  although  it  is  simply  the 
highest  point  in  a  broad,  plateau-like  body  of  nearly  horizontal  limestones. 
To  the  west  and  north  the  beds  incline  gently  toward  the  sage  brush  plain 
of  Antelope  Valley  and  the  broad  plain  west  of  the  Pinon  Range.  One  or 
two  of  the  more  deeply  eroded  canyons  offer  partial  exposures  of  the  beds, 
but  nowhere  any  continuous  sections  more  than  500  to  700  feet  in  thick- 
ness ;  yet  they  serve  to  show  similar  conditions  of  sedimentation  over  a  wide- 
spread area.  All  over  this  area,  at  several  horizons,  a  few  scattering 
fossils  may  be  found,  such  as  Atrypa  reticularis,  Strophomena  rkontboidaUs, 
Spirifera  pinonensis,  Stromatopora,  and  Chcetetes.  In  the  first  ravine  running 
up  to  Mahogany  Hills  from  Hay  Ranch  Valley,  the  limestones  afford 
such  large  numbers  of  corals,  partially  weathered  out,  that  the  locality 
would  well  repay  a  visit  by  anyone  specially  interested  in  the  study  of 
Devonian  fauna. 

Yahoo  canyon.— This  canyon  has  its  source  at  the  northern  end  of  Dry 

'U.  S.  Geol.  Explor.  40th  Par.,  vol.  ii,  Descriptive  Geology,  ]..  511. 


YAHOO  CANYON.  139 

Lake  and  is  the  only  one  of  the  principal  drainage  channels  of  the  Mahogany 
Hills  that  follows  a  north  and  south  course.  At  one  time  it  drained  the 
depressed  basin  of  Dry  Lake.  At  the  head  of  Yahoo  Canyon  a  small  out- 
burst of  rhyolite  forms  a  low  obscure  hill,  around  which  the  wagon  road 
passes  on  the  west  side.  A  few  hundred  feet  to  the  south  of  the  hill  is  a 
dike  of  similar  rock  about  100  feet  long  by  25  feet  wide.  This  rhyolite 
is  a  light  gray  rock,  weathering  brown,  and  carrying  a  few  macroscopic 
secretions  of  biotite,  sanadin,  and  quartz;  it  closely  resembles  the  rhyolite 
of  Browns  Canyon.  Yahoo  Canyon  presents  some  interest  as  being  the 
dividing  line  between  two  quite  different  types  of  orographic  structure;  on 
the  west  side  the  plateau-like  body  of  limestones  in  the  neighborhood  of 
Devon  and  Temple  Peaks  lies  gently  inclined  to  the  westward,  while  on  the 
east  side  the  limestones  have  been  uplifted  into  longitudinal  ridges  with  the 
structural  peculiarities  of  the  Pifion  Range.  In  general  the  canyon  may  be 
said  to  have  been  eroded  along  the  axis  of  an  anticlinal  fold,  although  this 
is  not  strictly  correct,  as  on  the  east  side  near  its  lower  end  a  sharp  anti- 
clinal ridge  exists,  which,  however,  dies  out  toward  the  head  of  the  canyon. 
The  structural  details  are  rather  intricate  and  were  by  no  means  carefully 
worked  out,  but  the  dips  and  strikes  indicated  on  the  map  (atlas  sheet  v.) 
show  this  anticlinal  structure  with  the  trend  of  the  ridges  agreeing  with  the 
course  of  the  canyon.  The  main  ridge  of  limestones  east  of  Yahoo  Canyon 
inclines  invariably  to  the  eastward  with  an  average  dip  of  about  35°  and 
with  a  strike  a  little  west  of  north,  maintaining  this  position  till  passing 
beneath  the  Carboniferous  rocks  which  everywhere  seem  to  overlie  them 
conformably.  The  ridge  is  made  up  of  monotonous  blue  massive  lime- 
stones characteristic  of  the  Upper  Nevada  epoch  as  seen  elsewhere,  espe- 
cially in  the  neighborhood  of  Signal  Peak  on  the  west  side  and  Newark 
Mountain  on  the  east  side  of  the  district.  On  the  east  side  of  Yahoo 
Canyon  a  most  interesting  collection  of  characteristic  species  was  made, 
consisting  largely  of  Upper  Devonian  corals.  Associated  with  them  occurs 
such  distinctive  species  as  Spirifera  disjuncta  and  the  widely  distributed 
Spirifera  glabra ;  a  fauna  indicating  a  higher  horizon  than  any  of  the  ex- 
amined beds  in  the  Mahogany  Hills  to  the  west.  A  list  of  the  fauna 
obtained  is  given  on  page  83.  Between  this  locality  and  the  Diamond 


140  GEOLOGY  OF  THE  EUKEKA  DISTRICT. 

Peak  quartzites,  paleontology  again  supports  structural  evidence,  the  organic 
forms  being  such  as  are  only  found  in  the  upper  horizons  or  mingled  with 
those  having  a  wide  vertical  range. 

Spanish  Mountain.— This  broad,  elevated  mass  of  Eureka  quartzite,  nearly 
two  and  one-half  miles  in  width,  lies  due  west  of  Prospect  Peak.  Its  struc- 
tural features  differ  from  those  of  any  other  area  of  the  Eureka  Mountains, 
.but  at  the  same  time  bear  some  resemblance  to  those  of  Grays  Peak,  both 
being  formed  of  strata  of  the  same  geological  age,  with  the  Lone  Mountain 
beds  resting  upon  their  slopes.  On  Spanish  Mountain  the  quartzites  dip 
away  in  every  direction  from  the  summit,  but  without  any  clearly  defined 
lines  of  bedding,  presenting  the  appearance  of  a  great  dome-shaped  body 
falling  away  on  all  sides.  This  quartzite  is  fractured  by  local  displace- 
ments, but  they  fail  to  bring  to  the  surface  any  underlying  Pogonip  beds, 
and  the  few  drainage  channels,  which  have  cut  one  or  two  narrow  gorges, 
still  lie  wholly  within  the  quartzite.  Over  this  dome-shaped  body  the  Lone 
Mountain  beds  undoubtedly  passed  at  one  time;  erosion,  however,  has 
worn  them  off  the  summit,  with  the  exception  of  two  small  patches,  which 
are  sufficient  to  establish  the  fact  that  the  upper  members  of  the  quartzite 
are  still  in  place  on  the  top  of  the  mountain.  Surrounding  the  quartzite 
on  all  sides  occurs  the  Lone  Mountain  limestone,  except  along  Spring  Valley, 
where  it  is  probably  obscured  by  recent  accumulations. 

Isolated  patches  of  limestone  in  the  valley  confirm  the  opinion  that 
the  Lone  Mountain  beds  extend  down  to  the  Spring  Valley  fault.  These 
limestones  cross  the  divide  connecting  Spanish  Mountain  with  Swiss 
Mountain  and  come  within  200  feet  of  the  summit  of  the  former.  Wherever 
observed,  the  limestones  rest  unconformably  upon  the  quartzite,  but,  as  they 
are  for  the  most  part  devoid  of  bedding  plane,  no  determination  can  be 
made  of  their  thickness.  Moreover,  the  line  between  the  Silurian  and 
Devonian  is  arbitrarily  drawn  and  rests,  as  elsewhere  in  the  district,  on 
lithological  distinctions  and  the  absence  of  evidence  of  life  in  the  lower 
rocks.  As  shown  on  the  map,  the  thickness  ascribed  to  the  Lone  Mountain 
beds  varies  greatly  at  different  localities,  but  there  is  no  doubt  that  the 
vertical  distance  between  Eureka  quartzite  and  limestone  characterized 
by  a  Devonian  fauna  actually  does  exhibit  great  variations  in  thickness. 


SPANISH  MOUNTAIN.  141 

The  horiiblende-andesite  body  on  the  edge  of  Dry  Lake  Valley,  at  the 
southwest  base  of  Spanish  Mountain,  will  be  discussed  in  the  chapter 
devoted  to  igneous  rocks,  which  form  a  most  important  group,  not  only  in 
themselves,  but  in  connection  with  similar  outbursts  in  Sierra  Valley  and 
elsewhere.  Here  at  Dry  Lake  they  present  a  marvelous  variety  in  color, 
density  and  texture,  but  on  careful  study  they  are  shown  to  be  closely 
related,  with  a  marked  similarity  in  mineral  and  chemical  composition. 
The  small  body  designated  on  the  map  as  dacite  is  simply  an  extreme  form 
of  the  larger  mass,  being  characterized  by  considerable  free  quartz  and 
biotite,  and  has  much  the  nature  of  a  pumice,  while  the  main  body  might 
be  designated  more  concisely  as  an  andesitic  pearlite. 

North  of  Spanish  Mountain,  as  elsewhere,  the  Lone  Mountain  lime- 
stones pass  gradually  into  those  of  the  Nevada  epoch,  and  with  this  change 
the  structural  features  of  the  region  assume  new  aspects,  quite  different 
from  the  rest  of  Mahogany  Hills  or  Fish  Creek  Mountains.  From  Brush 
Creek  northward  the  structure  is  that  of  a  simple  monoclinal  ridge, 
trending  about  north  40°  west,  with  a  dip  invariably  to  the  east.  Rising 
above  the  Quaternary  accumulations  along  the  east  base  of  the  ridge  in 
Spring  Valley,  at  sufficiently  frequent  intervals  to  prove  the  continuity  of 
strata,  occur  exposures  of  quartzite  beds,  conformably  overlying  the  lime- 
stones. As  the  latter  beds  bear  ample  testimony  of  their  Devonian  age  to 
the  very  summit,  the  siliceous  strata  have  been  referred  to  the  Diamond 
Peak  horizon  of  the  Carboniferous.  Brush,  Modoc,  and  Signal  peaks  are 
the  culminating  elevations  along  this  limestone  ridge,  which  stretches  north- 
ward all  the  way  to  The  Gate.  Along  the  west  base  of  these  peaks 
runs  the  Modoc  fault,  extending,  southward  from  Hay  Ranch  Valley,  near 
The  Gate,  till  lost  in  the  Lone  Mountain  limestones  west  of  Brush  Peak. 
This  fault  brings  up  the  Diamond  Peak  horizon  in  juxtaposition  with  the 
Devonian,  leaving  the  limestone  ridge  between  two  nearly  parallel  belts  of 
quartzite  of  the  same  age,  conformable  on  the  east  side,  but  unconfonuable 
on  the  west.  As  the  line  of  the  fault  follows  the  contact  between  two  dis- 
similar rocks  it  is  easily  traced.  North  of  Signal  Peak  erosion  has  worn 
out  a  deep  ravine  along  the  contact,  and  still  farther  southward  the  east 
drainage  of  lieilley  Creek  also  owes  its  origin  to  erosion  along  the  same 


142  GEOLOGY  OF  THE  EUEEKA  DISTRICT. 

fault  line.  The  Diamond  Peak  beds  may  be  represented  in  their  full 
development  near  The  Gate,  but  they  gradually  die  out  to  the  southward 
in  a  wedge-shaped  body  and  finally  disappear  altogether,  beyond  which  the 
fault  may  be  followed  for  a  considerable  distance,  with  Nevada  limestone 
walls  upon  both  sides.  This  long  body  of  Diamond  Peak  quartzite  rests 
conformably  upon  the  Nevada  limestone  to  the  westward,  both  series  of 
strata  dipping  uniformly  to  the  east,  We  have  here,  then,  a  duplication  of 
strata  made  up  of  the  Upper  Nevada  limestone,  overlain  by  the  Diamond 
Peak  quartzite.  Small  drainage  channels,  branches  of  Reilley  Creek  traverse 
the  quartzite,  affording  fair  cross  sections.  Numerous  minor  dislocations, 
at  right  angles  to  the  Modoc  fault,  trend  easterly  across  the  ridge,  dying 
out  in  the  plain  beyond,  but,  while  they  tend  to  break  up  the  uniformity 
of  structure,  do  not  cause  any  very  decided  dislocation  in  the  Nevada  lime- 
stones. 

Perhaps  the  best'  section,  the  one  showing  the  greatest  vertical  thick- 
ness across  the  Nevada  limestone,  may  be  found  on  the  ridge  north  of 
Modoc  Peak.  This  section  is  given  on  page  66.  Starting  in  at  the  Modoc 
fault  in  Reilley  Canyon,  nearly  due  west  of  Modoc  Peak,  it  crosses  the 
strata  nearly  at  right  angles  and  terminates  at  the  base  of  the  hills  in  Dia- 
mond Valley.  The  beds  strike  N.  50°  to  55°  W.,  measuring  about  5,400 
feet  in  thickness.  Just  north  of  Modoc  Peak  a  fossiliferous  shaly  limestone, 
200  feet  in  thickness,  crosses  the  ridge.  It  is  the  belt  designated  No.  3  of 
the  section,  and  is  the  equivalent  of  the  rich  fossiliferous  shale  which  has 
yielded  such  an  abundant  Devonian  fauna  at  several  localities  in  the  Dis- 
trict, notably,  at  Brush  Peak,  about  2  miles  southward.  Higher  up  in  the 
strata,  corals  of  the  middle  and  upper  horizons  were  obtained,  but  nowhere 
immediately  along  the  line  of  the  section  was  any  special  fossiliferous  zone 
recognized. 

Both  north  and  south  of  the  line  of  the  section  the  strata  are  easily 
traceable,  striking  obliquely  across  the  ridge,  the  upper  horizons  being 
developed  on  Signal  Peak  and  the  lower  on  Modoc  and  Brush  Peaks.  Just 
below  the  summit  of  Brush  Peak  the  fossiliferous  shale  belt,  which  is  here 
about  150  feet  in  width,  determines  the  position  of  the  beds  without  ques- 
tion. It  is  at  this  locality  that  the  shales  have  furnished  such  an  excellent 


METAMORPHOSED  SANDSTONES.  143 

opportunity  for  the  collection  of  a  Devonian  fauna.  The  few  hours  spent 
here  gave  promise  of  an  abundant  harvest  if  time  would  permit  of  a  dili- 
gent search.  From  this  shale  belt  the  limestones  pass  down  into  the  Lone 
Mountain  series,  the  hill  lying  between  Brush  Peak  and  Spanish  Mountain 
being  formed  of  the  latter  beds. 

Metamorphosed  sandstones.— Interstratified  in  the  Nevada  limestone  of  this 
ridge  occur  numerous  bands  of  fine  grained  sandstones  with  their  bedding 
planes  parallel  to  the  inclosing  rock.  Some  of  them  may  be  traced  for  over 
a  mile  without  interruption,  rarely  exceeding  50  feet  in  thickness,  but  most 
of  them  only  a  few  feet  in  width.  They  are  shown  in  the  section  north  of 
Modoc  Peak  occurring  at  varying  intervals  throughout  nearly  1,000  feet  of 
limestones. 

Instances  of  sandstones  in  limestones  are  common  enough  and  would 
call  for  no  special  comment  but  for  the  fact  that  here  they  have  undergone 
considerable  alteration,  and  as  the  original  material  was  more  or  less 
impure,  they  have  developed  under  dynamic  influences  a  crystallization 
and  structure  of  a  micro-granite.  All  of  these  sandstones  show  alteration, 
but  at  the  same  time  exhibit  remarkable  transitions  from  a  normal  sand- 
stone to  a  rock  closely  resembling  a  cryptocrystalline  granite.  The  quartz 
grains  are  granitoid  in  structure,  and  do  not  show  the  action  of  water  usually 
seen  in  a  compact  sandstone  made  up  from  the  disintegrated  material 
derived  from  an  older  rock.  Accompanying  these  quartz  grains  are  flakes 
of  muscovite  with  some  ferrite  and  calcite.  It  is  evident  that  the  beds  have 
undergone  a  marked  change  since  they  were  originally  laid  down.  That 
these  rocks  are  of  sedimentary  origin  no  one  would  question,  yet  they  are 
associated  with  others  which  have  undergone  so  great  an  alteration  that  they 
present  many  structural  features  of  igneous  rocks.  The  transition  from 
undoubted  sandstone  to  the  highly  metamorphosed  beds  shows  every  stage 
of  gradation  and  it  is  impossible  not  to  see  the  close  relationship  existing 
between  them.  In  the  more  highly  altered  rocks  may  be  observed  well 
developed  feldspars,  both  orthoclase  and  plagioclase.  Most  of  the  feldspars, 
however,  have  undergone  decomposition,  and  are  accompanied  by  calcite 
and  other  secondary  products.  Singularly  enough,  some  of  the  more  crys- 
talline bodies  exposed  along  the  west  sides  of  Signal  and  .Modoc  peaks  attain 


144  GEOLOGY  OF  THE  EUKEKA  DISTRICT. 

a  much  greater  width.  In  one  instance  the  rock  measures  about  200  feet 
across  its  broadest  development,  but  diminishes  rapidly  to  only  a  few  feet. 
Here  it  loses  its  distinctive  features  as  a  sedimentary  bed,  and,  on  the  con- 
trary, appears  to  cut  across  the  limestones,  suggesting  an  intrusive  dike. 
That  these  nearly  identical  rocks  should,  in  some  cases,  have  the  charac- 
teristics of  sedimentary  deposits,  and  in  others  those  of  an  intrusive  dike, 
is,  to  say  the  least,  most  remarkable  ;  but,  after  a  study  in  the  field  of  their 
mode  of  occurrence,  no  other  conclusion  seems  reasonable  than  that  they 
are  similar  rocks  which  have  undergone  various  degrees  of  metamorphism. 
These  occurrences  have  no  special  bearing  upon  the  history  of  the  sedimen- 
tary strata,  as  they  occupy  very  limited  areas  in  the  limestones,  and  perhaps 
still  less  upon  the  history  of  the  Tertiary  volcanic  outbursts  of  the  Eureka 
region.  They  are  well  worthy  an  investigation,  and  Mr.  Iddings,  in  his 
chapter  on  the  microscopical  petrography  of  the  crystalline  rocks,  has 
devoted  considerable  space  to  a  discussion  of  the  phenomena  which  these 
rocks  exhibit. 

signal  Peak.— On  this  peak  the  limestones  belong  exclusively  to  the  Upper 
Nevada  horizon,  being  massive  grayish  black  rocks,  distinctly  bedded. 
They  dip  northeast  about  35°.  The  fauna  is  characterized  by  Upper 
Devonian  corals,  associated  with  species  found  all  the  way  through  the 
Nevada  epoch.  North  of  Reilley  Canyon  the  beds  dip  eastward  at  a  .still 
lower  angle,  throwing  the  overlying  quartzite  to  the  east,  out  toward 
the  valley.  On  the  summit  of  the  ridge  north  of  the  last  named  canyon 
occur  Syringopora  hisingeri,  Bellerophon  mtera,  and  other  more  common 
forms,  the  beds  carrying  occasional  corals,  without  being  confined  to  any 
special  horizon. 

The  Gate.— At  The  Gate  occurs  a  marked  change  in  the  structure  of  the 
region.  The  ridge,  which  from  Brush  Peak  northward  maintains  a  fairly 
uniform  course,  here  undergoes  an  abrupt  break,  trending  off  more  to 
the  west,  and  at  the  same  time  the  entire  mountain  mass  north  of  The 
Gate  has  been  thrust  eastward,  bringing  the  beds  on  opposite  sides  of 
the  break  unconformably  against  each  other.  The  Gate  is  a  deep,  narrow 
gorge,  cutting  completely  through  the  ridge  along  the  line  of  the  disloca- 
tion. It  cuts  down  to  the  very  base  of  the  range,  draining  the  broad 


ABSENCE  OF  WHITE  PINE  SHALE.  145 

desert  region  of  Hayes  Valley  out  into  Diamond  Valley.  On  the  south 
side  of  The  Gate  the  beds  strike  N.  20°  W.,  dipping  20°  easterly,  but  on 
the  north  side  they  strike  N.  55°  W.,  with  a  dip  increased  to  30°  easterly. 
Owing  to  the  thrust  which  forced  the  beds  toward  the  east  the  walls  on  the 
south  side  belong  mainly  to  the  Diamond  Peak  quartzite,  while  those  on  the 
north  side  are  formed  of  a  bold  cliff  of  Nevada  limestone.  The  sections 
across  the  strata  on  opposite  sides  of  the  gorge  are  readily  correlated  by 
structural  features  confirmed  by  paleontological  evidence.  Fortunately, 
just  beneath  the  Diamond  Peak  beds  south  of  The  Gate  a  fauna  character- 
istic of  the  Upper  Nevada  limestone  occurs  in  the  low  ridge  near  the  west 
entrance  to  the  pass.  There  is  exposed  here  a  thickness  of  1,000  feet  of 
the  upper  limestones.  The  underlying  beds  are  dark  gray  in  color,  with 
poorly  preserved  fossils,  followed  by  a  black  band  bearing  many  large 
Stromatopora  and  other  corals.  Interstratified  in  these  limestones  are  several 
quite  shaly  beds,  seldom  more  than  1  foot  in  thickness.  These  gray  beds 
are  followed  by  a  belt  of  distinctly  stratified  black  limestones,  weathering 
a  light  color,  and  yielding  numerous  corals.  Above  this,  again,  are  thinly 
bedded,  dense  limestones,  extending  up  to  the  overlying  quartzites.  In 
these  latter  beds  occur  the  Upper  Devonian  fauna  already  mentioned. 
Conformably  overlying  the  limestones  occurs  a  broad  belt  of  Diamond 
Peak  beds,  forming  the  wall  along  the  south  side  of  The  Gate  and  extend- 
ing in  low,  round,  monotonous  hills  out  to  Diamond  Valley-  The  cliffs  on 
the  north  side  of  The  Gate  expose  about  500  feet  of  massive,  dark  lime- 
stones, passing  into  shaly  and  fissile  beds  2  or  3  feet  in  thickness.  A  rich 
and  varied  fauna  from  this  locality  will  be  found  published  in  full  on 
page  83.  The  locality  would  well  repay  a  more  diligent  and  careful 
search. 

Absence  of  white  Pine  shale.— On  both  sides  of  the  gorge  the  overlying 
siliceous  beds  are  much  the  same,  the  base  of  the  series  being  made  up  of 
quartzites,  interbedded,  impure  sandstones,  compact,  dense  argillites,  fine 
conglomerates,  and  black  cherty  layers,  rapidly  passing  into  purer  quartz- 
ites. On  the  south  side  the  black  cherty  belts  present  a  greater  thickness 
and  are  not  confined  to  the  base  of  the  horizon. 

It  will  be  noticed  that  no  mention  has  been  made  of  the  White  Pine 
MON  xx 10 


146  GEOLOGY  OF  THE  EUKEKA  DISTEIOT. 

shale,  which  on  the  east  side  of  the  Eureka  District  exposes  such  an  enor- 
mous thickness.  There  is  but  little  doubt  that  these  lower  beds  represent 
the  White  Pine  horizon,  but,  as  they  are  so  poorly  developed  as  compared 
with  the  shales  at  Newark  Mountain  and  so  difficult  to  trace  along  any 
definite  horizon,  they  have  been  omitted  on  the  geological  map.  No 
exposure  of  these  beds  was  seen  more  than  100  feet  in  thickness  and  in 
places  they  are  entirely  wanting.  It  would  seem  that  after  the  deposition  of 
the  limestones  the  conditions  here  were  more  favorable  for  purely  siliceous 
beds  than  at  Newark  Mountain,  and  that  the  transition  was  more  or  less 
rapid.  It  must  be  remembered  that  the  White  Pine  shale,  although  of  great 
thickness  at  White  Pine  and  on  the  east  side  of  the  district,  is  of  local 
occurrence,  never  as  yet  having  been  recognized  in  other  parts  of  the 
Great  Basin.  The  occurrence  in  the  argillites  just  south  of  The  Gate  of  a 
few  obscure  plant  remains  and  the  species  Discina  minuta  is  strong  evi- 
dence, taken  in  connection  with  their  stratigraphical  position,  that  these 
beds  represent  the  White  Pine  shale. 

The  Diamond  Peak  beds  which  overlie  the  limestones  on  the  north 
side  of  The  Gate  form  the  great  mass  of  Anchor  Peak,  showing  a  greater 
thickness  of  strata  than  the  same  horizon  exposes  in  the  Diamond  Range; 
the  explanation  being  found  in  the  argillites  of  the  White  Pine  shale  giving 
out  and  being  replaced  by  a  greater  development  of  siliceous  material. 
After  the  coming  in  of  the  siliceous  beds  north  of  The  Gate  the  quartzites 
stretch  for  nearly  a  mile  beyond  the  limits  of  the  map.  At  the  west  base 
of  Anchor  Peak  there  is  a  small  exposure  of  Devonian  limestones  dipping 
under  the  quartzites,  probably  extending  northward  along  the  west  base 
of  the  Pinon  Range. 

SILVERADO   AND   COUNTY   PEAK. 

This  mountain  block  is  mainly  outlined  by  profound  faults,  along  which 
igneous  rocks  of  varied  composition  have  burst  forth  in  vast  quantities, 
almost  completely  isolating  it  from  adjoining  sedimentary  regions.  On  the 
south  and  east  the  Quaternary  accumulations  of  Newark  and  Fish  Creek 
valleys  rest  against  the  base  of  the  hills  and  probably  in  a  large  degree 
conceal  eruptive  rocks  which  broke  out  along  the  edge  of  the  uplifted 


COUNTY  PEAK  REGION.  147 

mountain  mass,  but  nowhere  attained  any  considerable  elevation.  This 
mountain  block  is,  for  the  most  part,  made  up  of  sedimentary  beds  belong- 
ing to  the  Silurian  and  Devonian.  In  the  chapter  devoted  to  a  sketch  of 
the  general  geology  of  the  district  the  principal  features  of  this  region  are 
given,  and  in  the  chapter  on  the  Devonian  rocks  a  description  will  be  found 
of  the  Nevada  limestones,  together  with  some  discussion  upon  the  develop- 
ment of  the  Devonian  fauna,  as  shown  upon  Sentinel  Mountain,  Woodpeckers 
Peak,  and  Rescue  Hill.  Only  such  additional  facts  are  here  presented  as 
may  be  of  value  in  a  detailed  study  of  the  region  in  the  field  and  for  com- 
parative purposes  in  distant  areas  of  the  Great  Basin. 

County  Peak  Region.— The  Pinto  fault,  which  trends  approximately  parallel 
with  the  Hoosac  fault,  sharply  defines  this  block  on  the  west,  and,  like  the 
latter  fault,  is  probably  deflected  to  the  east  at  its  northern  end.  The  lowest 
rocks  exposed  by  the  fault  are  two  bodies  of  Eureka  quartzite,  one  imme- 
diately at  the  base  of  Richmond  Mountain,  the  other  near  by,  but  separated 
from  it  by  the  tuffs  of  Hornitus  Cone.  The  first  exposure  is  so  completely 
surrounded  by  igneous  rocks  that  there  is  nothing  to  indicate  its  geological 
position  but  Hthological  habit  and  proximity  to  the  second  and  larger  bod}', 
the  age  of  which  is  clearly  determined  by  overlying  Lone  Mountain 
beds.  At  its  northern  end  the  quartzite  of  this  larger  body  forms  a  broad- 
topped  hill  nearly  500  feet  in  height,  with  the  beds  inclined  a  few  degrees 
to  the  east.  As  regards  their  Hthological  habit,  they  could  not  be  distin- 
guished from  the  corresponding  beds  along  the  Hoosac  fault  or  those  in  the 
region  of  Grays  Peak. 

Along  the  Pinto  fault  the  quartzite  is  exposed  for  nearly  a  mile,  thin- 
ning out  in  a  wedge-shaped  body,  and  replaced  by  the  Lone  Mountain 
limestone,  which,  in  tuni,  gives  way  to  the  Nevada  limestone,  the  latter 
forming  the  fault  wall  opposite  Dome  Mountain.  Erosion  has  worn  out 
a  deep,  narrow  ravine  along  the  displacement,  with  the  Carboniferous  lime- 
stone, admirably  shown  on  one  side,  dipping  westerly,  at  angles  never  less 
than  60°,  and  the  Lone  Mountain  limestone  of  the  Silurian  equally  well 
shown  on  the  other  side,  dipping  easterly,  but  inclined  at  low  angles, 
seldom,  if  ever,  exceeding  20°. 

The  canyon  wall  is  cut  out  of  the  Lone   Mountain  beds,  but  on  the 


148  GEOLOGY  OF  THE  EUEEKA  DISTRICT. 

steep  lull  slopes  they  give  way  to  the  Nevada  limestones,  which  continue 
eastward  across  the  entire  width  of  the  mountains  till  they  are  lost  beneath 
the  lava  beds  of  Basalt  Peak.  County  Peak  (8,350  feet)  forms  the  culmi- 
nating point  of  this  broad,  elevated  mass  of  limestones,  all  the  beds  of  which 
strike  north  and  south  and  dip  easterly,  affording  an  excellent  cross-section 
over  5,200  feet  in  thickness,  with  the  Lone  Mountain  beds  at  the  base.  The 
sequence  of  rocks  shown  here  may  be  taken  as  a  typical  one  of  the 
Nevada  epoch  and  will  be  found  on  page  68,  in  a  chapter  devoted  to  the 
Devonian  rocks.  The  cross-section  E-F,  atlas  sheet  xm,  is  drawn  across  the 
summit  of  County  Peak,  and  gives  at  a  glance  the  structure  of  the  moun- 
tains, which  is  shown  better  here  than  to  the  south,  where  it  is  diffi- 
cult to  obtain  a  continuous  section  for  anything  like  the  same  distance  across 
the  strata  at  right  angles  to  their  strike.  Midway  on  the  ridge  connecting 
County  and  Woodpeckers  peaks,  about  200  feet  below  the  summit  and  3,000 
feet  above  the  base  of  the  limestone,  occurs  an  important  grouping  of  fos- 
sils exhibiting  the  most  complete  mingling  of  both  upper  and  lower  Devo- 
nian species  yet  found  in  the  district.  Radiating  from  County  Peak  iu  all 
directions  occur  numerous  narrow  gorges  scored  deeply  into  the  mountains, 
frequently  exposing  1,000  or  2,000  feet  of  strata  and  offering  excel- 
lent opportunities  for  detailed  studies  across  the  middle  Devonian  strata. 
These  gorges  are  the  source  of  the  two  drainage  channels  that  encircle 
Richmond  Mountain,  finally  running  out  into  Diamond  Valley.  North  of 
County  Peak  toward  Richmond  Mountain,  the  limestones  are  characterized 
by  a  development  of  siliceous  beds,  aggregating  a  thickness  of  over  100  feet 
and  rising  in  bold,  rugged  outcrops  above  the  otherwise  even  hill  slopes. 
Nowhere  else  were  similar  rocks  recognized  in  the  Devonian,  the  siliceous 
material  apparently  increasing  in  amount  toward  Richmond  Mountain, 
although  the  higher  horizons  maintained  their  normal  character.  It  is  only 
directly  west  of  County  Peak  that  the  upper  members  of  the  Nevada  lime- 
stone are  exposed,  the  basalts  concealing  more  and  more  of  the  beds  as  they 
approach  Richmond  Mountain.  In  this  area  north  of  County  Peak,  scarcely 
any  fossils  were  collected,  and  nowhere  any  grouping  of  species;  conse- 
quently no  locality  indicating  the  presence  of  organic  remains  is  marked 
upon  the  map.  It  is  proper  to  say,  however,  that  very  little  time  was  allot- 


SILVERADO  HILLS.  149 

ted  to  their  search,  but  it  seems  hardly  possible  that  they  are  absent,  as  occa- 
sional evidence  of  poorly  preserved  corals  was  noted  in  the  purer  limestones. 
Silverado  Hiiis.— South  of  Dome  Mountain  the  Lone  Mountain  strata  again 
come  in  along  the  Pinto  fault,  and  with  the  exception  of  occasional  breaks 
caused  by  overflows  of  both  rhyolite  and  basalt  continue  to  form  the  base 
of  the  sedimentary  beds  until  the  ridges  pass  beneath  the  deposits  of  Fish 
Creek  Valley.  These  rhy  elites  and  pumices,  with  the  glassy  basalts  break- 
ing through  them,  present  identical  features  with  those  found  in  the  basin 
south  of  Richmond  Mountain,  while  the  basalts  in  the  limestone  do  not 
differ  essentially  from  those  occurring  as  dikes  in  pyroxene-andesite. 

The  drainage  from  the  slope  of  Hoosac  Mountain  follows  a  southeast 
course  until  it  meets  the  upturned  Silurian  ridge  on  the  east  side  of  the 
Pinto  fault,  then  runs  south  across  the  Pinto  Basin,  where,  instead  of  con- 
tinuing southward  following  the  natural  grade  along  the  line  of  the  fault 
and  across  the  soft,  easily  eroded  pumices,  it  turns  abruptly  and  follows  a 
deep  channel  cut  clear  through  the  hard  rocks  of  English  Mountain,  finally 
running  southward  to  Fish  Creek  Valley.  The  divide  between  this  water 
course  and  the  broad  drainage  channel  running  southward  along  the  Pinto 
fault  and  also  emptying  into  Fish  Creek  Valley,  lies  only  a  few  feet  above 
the  level  of  the  two  stream  beds.  So  far  as  can  be  made  out  the  barrier 
between  the  streams  is  wholly  formed  of  recent  lavas.  It  is  similar  to  the 
case  mentioned  in  describing  the  drainage  of  Secret  Canyon,  where  the 
stream,  after  following  the  course  of  the  canyon  for  a  long  distance,  sud- 
denly crosses  the  upturned  ridge  of  Cambrian  and  Silurian  rocks,  avoiding 
the  low  and  insignificant  ridge  of  volcanic  material  which  blocks  the 
entrance  to  the  canyon.  The  cause  of  this  sudden  turn  in  the  course  of 
these  stream  beds  is  difficult  to  understand,  but  it  is  worthy  of  note  that  the 
drainage  channel  bi-eaking  through  English  Mountain  lies  nearly  due  east 
of  the  one  cutting  Hamburg  Ridge. 

The  Lone  Mountain  beds  are  not  so  uniformly  made  up  of  limestones 
as  the  corresponding  horizon  elsewhere.  Many  of  the  intercalated  strata 
resemble  the  underlying  Eureka  quartzites,  but,  as  the  latter  nowhere  carry 
any  considerable  layers  of  calcareous  material,  siu-h  a  reference  is  out  of 
the  question.  That  they  correspond  to  the  Lone  Mountain  horizon  there 


150  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

can  be  no  doubt,  the  only  difference  being  that  the  siliceous  beds  occur 
here  more  prominently  developed  than  on  the  west  side  of  the  Hoosac  fault 
with  the  friable  sandstones  altered  to  compact  quartzites.  Moreover,  they 
are  seen  to  pass  into  Nevada  limestones,  except  where  their  continuity  is 
broken  by  outbursts  of  basalt.  In  the  region  of  English  Mountain  this 
connection  is  in  no  way  disturbed  by  intrusive  material  and  the  transition 
into  the  Nevada  beds  maybe  readily  made  out.  Nevertheless,  there  occurs 
along  the  Pinto  fault  one  or  two  exposures  of  siliceous  beds  whose  geolog- 
ical position  it  is  difficult  to  determine.  One  of  these  is  found  east  of  the 
Pinto  Mill,  where  a  long,  narrow  ridge,  largely  made  up  of  quartzites,  dips 
from  25°  to  35°  to  the  east.  A  ravine,  which  cuts  through  this  ridge,  gives 
a  fair  idea  of  the  beds,  and  it  is  not  improbable  that  they  belong  to  the 
Eureka  horizon.  Another  instance  may  be  found  southeast  of  Pinto  Basin, 
near  the  place  called  The  Wells,  where  a  small  isolated  hill  occurs,  appar- 
ently a  faulted  mass  composed  of  white  vitreous  quartzite  with  intercalated 
bluish  gray  limestones.  Except  for  these  limestones  the  evidence  would 
point  quite  as  much  to  the  Eureka  quartzite  as  to  the  overlying  Lone  Moun- 
tain beds.  English  Mountain  offers  the  best  locality  for  a  sttidy  of  these 
Lone  Mountain  beds  to  be  found  on  the  east  side  of  the  district,  as  they 
show  a  gr.eat  thickness  of  strata  dipping  uniformly  eastward,  overlain  by 
the  lower  beds  of  the  Nevada  limestones.  The  base  of  English  Mountain 
is  formed  of  quartzites  and  sandstones,  followed  by  gray  limestone,  in  turn 
capped  by  brownish  red,  vitreous  quartzite.  The  latter  is  a  rough  and 
jagged  rock,  full  of  nodules  and  water-worn  cavities. 

On  the  south  side  of  the  Silverado  Hills  the  Silurian  rocks  rise  above 
the  pumices  and  tuffs  that  follow  the  base  of  the  hills  and  in  a  large  degree 
conceal  the  sedimentary  beds.  Here  the  limestones  have  gradually  changed 
their  strike  and  dip  and  lie  inclined  to  the  northward  with  the  great  body 
of  Devonian  limestone  that  forms  the  bold  escarpment  of  Red  Ridge  resting 
upon  them.  Continuing  eastward,  the  limestones  gradually  swing  around 
until  they  assume  a  westerly  dip,  forming  a  synclinal  fold,  with  those  of 
English  Mountain.  This  Red  Ridge  escarpment  offers  excellent  vertical 
sections  of  the  middle  portions  of  the  Nevada  limestones,  and  the  variegated 
red,  gray,  and  brown  belts,  with  the  interbedded  sandstones,  may  be  traced 


PACKEK  BASIN.  15] 

for  long  distances  from  one  mountain  to  another.  In  this  way  it  becomes 
an  easy  matter  to  correlate  strata  in  such  blocks  as  Island  and  Leader 
mountains  and  Sugar  Loaf.  The  deep  gorges  penetrating  the  limestones 
afford  grand  exposures.  Sugar  Loaf  offers  one  of  the  best  points  of  view 
for  gaining  a  clear  understanding  of  the  synclinal  structure  of  the  Silverado 
Hills,  the  characteristic  belts  of  sandstones  and  mottled  limestones  being 
readily  traceable  from  an  easterly  to  a  westerly  dip.  The  summit  of  Sugar 
Loaf  is  formed  of  Upper  Devonian  strata,  with  abrupt  escarpments  on  all 
sides.  At  the  east  base  of  this  isolated  mountain,  the  Rescue  Canyon  fault 
may  be  traced  crossing  the  ridge  between  the  head  of  Rescue  Canyon  and 
the  faulted  block  of  White  Pine  shales  at  Charcoal  Canyon.  From  Sugar 
Loaf  northward  to  Packer  Basin  all  the  limestones  on  the  west  side  of  the 
fault  dip  westerly,  the  fault  following  the  line  of  contact  between  the 
Nevada  limestone  and  the  White  Pine  shale.  Opportunities  for  observing 
these  westerly  dipping  beds  may  be  found  in  Charcoal  and  Ox  Bow  canyons, 
the  streams  which  cut  the  ravines  crossing  the  strata  nearly  at  right  angles 
to  their  strike. 

Packer  Basin.— Packer  Basin  is  a  small  depressed  block  of  Nevada  lime- 
stone lying  between  the  northern  end  of  the  main  ridge  and  the  broad 
basalt  table,  the  abrupt  wall  of  the  latter  shutting  in  the  basin  on  the  north. 
As  the  basin  lies  on  the  very  edge  of  a  broad  volcanic  field,  it  has  naturally 
undergone  a  good  deal  of  dislocation,  and  is  much  broken  up  by  pumices 
and  tuffs,  which  partly  fill  the  basin,  having  poured  out  along  a  fissure 
on  the  west  side  of  the  faulted  block.  It  is  interesting  to  see  here  the  same 
association  of  pumices  and  tuffs,  followed  by  a  later  outburst  of  basalt,  in 
all  respects  similar  to  those  occurrences  seen  in  so  many  other  places 
bordering  the  uplifted  block.  The  limestone  still  maintains  the  north  and 
south  strike  and  westerly  dip  of  the  main  ridge  to  which  it  really  belongs. 
Its  chief  interest  lies  in  the  finding  in  a  massive  blue  limestone  a  fauna 
characteristic  of  a  somewhat  higher  horizon  than  those  observed  at  Wood- 
peckers and  Basalt  peaks.  Additional  interest  is  derived  from  the  disappear- 
ance of  the  Rescue  fault  and  the  accompanying  White  Pine  shales  beneath 
the  basalts. 


152  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

Rescue  Hiii.— Scarcely  any  mention  need  be  made  here  of  this  locality  as 
the  essential  structural  features  and  the  list  of  species  obtained  in  the  upper 
beds  have  been  given  in  the  chapter  describing  the  Devonian  rocks.  The 
hill  is  a  block  of  limestone  faulted  over  1,000  feet  below  its  true  strati- 
graphical  position.  It  lies  in  the  angle  formed  by  the  intersection  of  the 
Rescue  and  Silverado  faults.  The  beds  lie  inclined  at  a  very  low  angle 
presenting  an  excellent  section  for  comparative  purposes  with  beds  found 
elsewhere.  Owing  to  the  faulting  of  this  block  the  variegated  beds  of  Red 
Ridge  can  not  be  followed  on  Rescue  Hill,  but  to  the  north  they  are 
easily  traceable  on  Island  Mountain  and  Sugar  Loaf. 

Century  Peak  Ridge.— Rescue  Canyon  severs  the  Century  Peak  Ridge  from 
the  main  body  of  Silverado  Hills,  a  separation  which  is  intensified  by  the 
rhyolitic  outbursts  along  the  line  of  the  canyon.  Structurally  the  country 
east  of  the  canyon  differs  in  a  most  striking  manner  from  Red  Ridge  and 
Rescue  Hill,  the  horizontal,  plateau-like  character  of  the  former  giving  way 
to  a  narrow  ridge  with  steep  slopes.  This  ridge,  of  which  Century  Peak  is 
the  highest  point,  presents  a  sharp,  anticlinal  fold,  the  beds  dipping  away 
from  the  axis  at  angles  varying  from  70°  to  80°.  The  axis  of  the  fold 
follows  closely  the  crest  of  the  ridge,  with  a  strike  approximately  north 
and  south.  On  the  summit  of  Century  Peak  occurs  one  of  the  many 
intercalated  beds  of  quartzite  found  in  the  Nevada  limestone,  and  here 
forms  the  greater  part  of  the  west  slope,  extending  down  the  ridge  nearly 
to  the  line  of  rhyolite.  Just  where  this  quartzite  belt  belongs  in  the  lime- 
stone was  not  determined,  but  the  entire  uplift  is  of  Upper  Devonian  age, 
as  is  shown  by  the  lithological  character  of  the  beds.  No  fossils  identify- 
ing any  special  horizon  were  obtained,  but  those  found  were  forms  having  a 
wide  vertical  range,  such  as  Atrypa  reticularis.  The  corals  belong  to  the 
upper  portion  of  the  limestone  and,  although  too  obscure  for  specific  iden- 
tification, closely  resemble  the  forms  found  in  the  limestones  at  the  northern 
end  of  the  Mahogany  Hills.  Along  the  line  of  the  Silverado  fault  the 
rocks  give  evidence  of  considerable  disturbance  and  folding  with  abrupt 
flexures  and  breaks.  For  the  greater  part  of  the  distance  along  the  canyon 
the  Nevada  limestone  may  be  seen  south  of  the  fault,  with  the  White  Pine 
shale  on  the  north  or  opposite  side  of  the  gorge  resting  unconformably 


ALHAMBEA  HILLS.  153 

against  it.  On  the  north  side  of  the  Silverado  fault,  between  the  White 
Pine  shale  and  the  rhyolites  occurring  at  the  head  of  Rescue  Canyon,  is  a 
triangular  block  of  limestone  inclined  to  the  east.  This  block  of  limestone 
lies  on  the  east  side  of  the  Rescue  fault,  conformably  underlying  the  White 
Pine  shale  and  offering  ample  structural  evidence  that  it  belongs  to  the 
highest  beds  of  the  Nevada  horizon.  The  amount  of  faulting  along  Silver- 
ado Canyon  has  never  been  determined,  but  probably  does  not  exceed  a  few 
hundred  feet,  which  is  additional  evidence  that  the  Century  Peak  beds 
belong  to  the  upper  portion  of  the  Devonian.  South  of  Century  Peak  there 
is  a  decided  break  in  the  strata  and  the  entire  limestone  ridge  dips  off 
toward  Fish  Creek  Valley,  with  a  northeast  and  southwest  strike. 

Aihambra  Hiiis.— The  low  ridge  of  limestone  designated  as  the  Alhambra 
Hills  lies  to  the  east  of  the  Century  Peak  ridge  and  is  connected  with  the 
latter  by  a  continuous  body  of  limestone.  North  of  this  connecting  ridge 
Quaternary  deposits  lie  between  these  hills  and  Century  Peak  ridge, 
but  they  are  of  no  great  thickness  and  undoubtedly  overlie  a  depressed 
area  of  limestone.  Alhambra  Hills  rise  but  a  few  hundred  feet  above 
the  plain.  They  present  a  dull,  monotonous,  arid  aspect,  with  but  few 
scattered  trees  and  without  soil.  The  limestones  belong  to  the  upper 
members  of  the  Nevada  horizon  and  are  massive,  distinctly  bedded,  grayish 
blue  rocks.  But  little  time  was  devoted  to  the  search  for  fossils,  but  such 
as  were  found  denoted  the  upper  beds  of  the  Nevada  and  were  mostly 
corals  similar  to  those  found  in  the  neighborhood  of  Century  Peak,  associ- 
ated with  the  ever  present  Atrypa  reticularis.  Beyond  this  identification  of 
the  age  of  the  beds  the  Alhambra  Hills  present  no  special  geological  in- 
terest. A  few  mineral  veins  penetrate  the  limestone,  but  so  far  as  known 
are  unaccompanied  by  rhyolite  intrusions.  The  latter  rock,  while  it  prob- 
ably encircles  the  Alhambra  Hills,  does  not  appear  to  enter  the  limestone 
body. 

White  Pine  Shale  Area.— There  is  little  that  need  be  said  about  this  area  in 
addition  to  the  observations  presented  elsewhere  in  discussing  the  geological 
position  and  the  paleontological  evidence  of  the  age  of  the  White  Pine 
shale.  On  page  81  will  be  found  a  description  of  the  strata  across  the 
entire  thickness  of  shales  and  sandstones,  at  least  until  they  are  overlain 


154  GEOLOGY  OF  THE  EU11EKA  DISTRICT. 

by  Quaternary  deposits.  They  measure  over  2,000  feet.  This  section  was 
made  east  of  Sugar  Loaf,  where  the  underlying  limestones  are  exposed, 
passing  conformably  beneath  the  broadest  expansion  of  overlying  shales. 
The  occurrence  of  this  limestone  is  exceedingly  fortunate,  as  upon  it 
rests  the  evidence  of  the  position  of  the  overlying  shales,  whereas,  north  of 
Charcoal  Canyon  no  limestones  occur  beneath  the  shale,  and  as  the  beds 
trend  to  the  northwest  with  a  greater  angle  than  the  course  of  the  Rescue 
fault,  the  lower  strata  are  cut  off  along  the  line  of  the  displacement.  Direct 
evidence  is  wanting  of  the  precise  position  of  the  beds  lying  next  the 
fault.  From  Silverado  Canyon  northward  to  Packer  Basin  the  strata  dip 
uniformly  eastward.  Charcoal  Canyon,  Ox  Bow  Canyon,  and  the  other 
drainage  channels  traversing  the  formation,  fail  to  give  any  good  sections 
across  the  beds,  as  the  valleys,  though  broad,  are  extremely  shallow,  with 
the  underlying  rocks  more  or  less  covered  with  soil  and  gravel,  derived  from 
the  disintegration  of  the  friable  interbedded  sandstones.  The  stream  bed 
coming  from  Packer  Basin  has  eroded  somewhat  more  deeply  into  the  shale 
formation,  the  beds  lying  more  highly  inclined,  but  shortly  after  leaving 
the  mountains  it  enters  the  tuffs  which  overlie  the  shales. 

cuff  Hills.— South  of  Silverado  Hills,  and  separated  from  them  by  the 
broad  expanse  of  Fish  Creek  Valley,  lies  a  low  ridge  designated  Cliff  Hills 
on  account  of  the  mural-like  escarpment  which  they  present  to  the  Quater- 
nary plain.  These  hills  have  no  direct  topographical  connection  with  the 
Eureka  Mountains  and  are  referred  to  here  only  because  they  happen  to 
come  in  on  the  southeast  corner  of  the  map.  By  reference  to  atlas  sheet 
xn,  their  relations  to  the  Eureka  Mountains  may  be  seen  at  a  glance.  Geo- 
logically they  are  of  great  interest,  as  the  White  Pine  shale,  which  has 
been  recognized  over  such  limited  areas,  occurs  here  under  conditions  simi- 
lar to  those  found  east  of  Sugar  Loaf.  Low  undulating  ridges  of  shale  and 
sandstone  formed  of  westerly  dipping  beds  pass  beneath  a  broad,  flat-topped 
body  of  pyroxene-andesite.  It  is  this  andesite  which  gives  the  cliff-like 
appearance  to  the  hills,  the  dark  bare  rocks  presenting  a  forbidding  aspect 
as  they  rise  above  the  desert  valley.  In  then*  mode  of  occurrence  and 
petrographical  habit  these  andesites  closely  resemble  those  of  Richmond 
Mountain,  and  show  the  same  modification  in  color,  density,  and  chemical 


NEWAEK  MOUNTAIN.  155 

composition;  in  mineral  composition  they  are  identical.  These  resemblances 
are  borne  out  by  microscopical  investigation,  the  differences  in  structure  in 
Richmond  Mountain  finding  their  counterpart  in  Cliff  Hills. 

Cropping  out  beneath  the  andesites  at  the  north  end  of  the  hills  are  three 
small  exposures  of  gray  limestones,  only  one  of  which  is  represented  on  the 
map.  It  dips  westerly  at  an  angle  of  15°  and  strikes  nearly  north  and 
south.  No  evidence  of  the  age  of  these  limestones  could  be  obtained,  but 
from  their  proximity  to  the  White  Pine  shale  and  their  general  resemblance 
to  the  Devonian  rocks  of  the  Silverado  region,  they  have  been  referred  to 
the  Nevada  limestone.  In  the  White  Pine  shale  a  few  fragmentary  plant 
remains  were  procured,  none  of  which  were  sufficiently  well  preserved  to 
admit  of  identification,  although  they  bear  the  closest  resemblance  to  the 
plants  found  elsewhere  at  this  horizon. 

DIAMOND   RANGE. 

Few  of  the  narrow  longitudinal  ridges  in  central  Nevada  form  so  prom- 
inent a  physical  feature  as  the  Diamond  Range.  Only  the  southern  end, 
however,  comes  within  the  limits  of  the  Eureka  District,  but  here  it  is  so 
intimately  connected  with  the  County  Peak  and  Silverado  uplift  as  to  form 
a  part  of  the  same  geological  region. 

Diamond  Peak,  the  highest  elevation  in  the  range,  is  situated  just 
within  the  limits  of  the  survey,  although  the  north  and  east  slopes  lie 
beyond  the  boundaries  of  the  map.  In  a  study  of  the  sedimentary  rocks  of 
the  Eureka  district,  this  peak  is  of  the  highest  interest,  showing  the  rela- 
tionship between  the  Devonian  and  Carboniferous  beds  in  a  manner  unsur- 
passed elsewhere  in  the  Great  Basin,  and  at  the  same  time  carrying  the 
Paleozoic  section  nearly,  if  not  quite,  to  the  top  of  the  Upper  Coal-meas- 
ure limestone. 

Newark  Mountain,— As  seen  from  the  east,    Newark  Mountain  present*  a 

bold  front  of  blue  limestone  rising  nearly  2,000  feet  above  Newark  Valley, 
the  upper  1,000  feet  an  abrupt  cliff,  followed  by  a  highly  inclined  slope 
to  the  plain.  Along  the  summit  it  is  a  narrow  ridge  3  miles  in  length,  fall- 
ing off  gradually  toward  the  west  in  strong  contrast  with  the  opposite  side. 
In  structure,  Newark  Mountain  is  an  anticlinal  fold  whose  axis  may  be 
traced  all  along  the  base  of  the  cliff,  the  eastern  side  of  the  arch  having 


156  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

dropped  about  1,000  feet,  causing  a  picturesque  escarpment.  It  is  a  fine 
example  of  a  limestone  wall  formed  by  a  displacement.  The  easterly 
inclined  beds,  begining  at  the  base  of  the  cliff  with  a  dip  from  15°  to  25°, 
gradually  fall  away  with  a  less  and  less  angle,  stretching  in  low  broken  hills 
and  knolls  far  out  toward  the  plain.  Along  the  face  of  the  cliff  on  the  west 
side  of  the  anticline  the  strata  incline  into  the  mountain,  arching  over  from 
an  angle  of  25°  on  the  crest  of  the  ridge  to  55°  along  the  western  base  in 
Hayes  Canyon.  At  the  southern  end  of  the  ridge  the  beds  rise  steeply 
out  of  the  Quaternary  plain  along  the  line  of  an  east  and  west  fault. 
They  strike  a  few  degrees  east  of  north,  gradually  curving  more  and  more 
to  the  east,  coinciding  approximately  with  the  trend  of  the  i-idge  until  at  the 
northern  end  they  fall  away  toward  Newark  Valley  and  pass  beneath  the 
east  base  of  Diamond  Peak.  The  limestones  of  Newark  Mountain  belong 
to  the  upper  portion  of  the  Nevada  Devonian.  They  are  usually  dark  blue 
and  gray  in  color  and  distinctly  bedded.  It  is  estimated  that  there  are 
exposed  on  the  mountain  about  3,500  feetof  these  upper  Nevadalimestones, 
which  would  carry  the  beds  down  nearly  to  the  middle  of  the  formation 
They  may  be  correlated  readily  with  the  limestones  of  Silverado  Hills  by 
the  sequence  of  strata  and  by  their  physical  habit.  Their  stratigraphical 
position  is  determined  without  doubt  by  the  overlying  White  Pine  shale  in 
Hayes  Canyon,  the  contact  between  the  two  formations  being  easily  trace- 
able for  miles,  all  the  way  from  the  entrance  to  the  canyon  around  to  the 
northern  base  of  Diamond  Peak.  Paleontological  evidence  confirms  other 
evidences  by  the  finding  of  upper  Devonian  species  in  several  localities  in 
two  distinct  horizons,  one,  near  the  summit  of  the  limestones  along  the  west 
base  of  the  mountain,  the  other,  several  hundred  feet  lower  down  in  light 
gray,  somewhat  shaly  beds  on  the  south  side  of  Milk  Canyon.  Fossils 
may  also  be  obtained  near  the  summit  of  the  mountain.  A  list  of  the 
species  obtained  from  both  horizons  will  be  found  in  the  chapter  devoted  to 
the  discussion  of  the  Devonian  rocks,  and,  while  they  both  contain  specific 
forms  having  a  wide  vertical  range,  they  are  characterized  by  types  found 
only  in  the  upper  Devonian.  The  species  Beyrichia  occidentalis,  obtained  just 
below  the  White  Pine  shale  in  Hayes  Canyon,  occurs  on  the  east  side  of 
the  mountain  1,000  feet  or  more  below  the  summit;  it  has  also  been  identi- 


DIAMOND  PEAK.  157 

fied  from  the  top  of  Telegraph  Peak  at  White  Pine,  where  it  also  occurs 
not  far  below  the  base  of  the  shale. 

At  the  summit  of  the  Nevada  beds  a  reddish  gray,  impure  limestone 
passes  gradually  into  the  black,  argillaceous  shales  of  the  White  Pine 
series,  the  contact  between  the  two  formations  being  admirably  shown  all 
along  Hayes  Canyon  at  the  base  of  Newark  Mountain.  The  drainage 
channel  marks  closely  the  line  of  contact.  Hayes  Canyon  lies  wholly  in 
the  shales,  erosion  having  carved  out  of  them  a  broad  valley,  similar  in 
topographical  structure  to  Secret  Canyon,  between  the  Prospect  Mountain 
and  Hamburg  limestones.  Upon  one  side  of  Hayes  Canyon  rises  a  wall  of 
dark  blue,  Devonian  limestone,  and  on  the  other  light  blue  and  gray  Car- 
boniferous limestone.  At  the  summit  of  Hayes  Canyon  the  shales  follow- 
ing the  course  of  the  limestones  of  Newark  Mountain  trend  off  to  the  north- 
east and  rapidly  pass  under  Diamond  Peak.  The  relationship  between  the 
shales  and  the  Diamond  Peak  quartzite  may  be  best  studied  along  the  base 
of  Bold  Bluff,  the  former  being  seen  to  dip  conformably  beneath  the 
quartzites  at  an  angle  of  30°. 

Diamond  Peak.— The  summit  of  Diamond  Peak  attains  the  highest  eleva- 
tion of  any  point  within  the  limits  of  this  survey,  reaching  an  altitude 
above  sea  level  of  10,637  feet.  From  Newark  Valley  it  rises  for  over  4,000 
feet  with  an  almost  unbroken  slope  to  the  summit.  No  peak  commands 
a  more  favorable  view  for  a  study  of  the  relationship  between  the  topo- 
graphical configuration  and  geological  structure  of  the  country.  The 
structure  of  the  peak  is  that  of  a  sharp,  synclinal  fold,  the  axis  of  which, 
striking  northeast  and  southwest,  lies  along  the  crest  of  the  ridge.  The 
westerly  dipping  beds  form  the  entire  eastern  slope  of  the  peak,  exhibiting 
a  great  thickness  of  Devonian  and  Carboniferous  rocks.  At  the  base  of 
the  peak,  just  outside  the  limits  of  the  map,  the  Nevada  limestone  comes 
in,  overlain  by  a  broad  belt  of  black  shales,  which  form  the  lower  slopes, 
but,  as  denudation  has  worn  them  smooth,  they  present  rather  a  monoto- 
nous aspect.  Following  the  shales  are  the  Diamond  Peak  quartzites,  in  rough 
and  rugged  ridges  and  bold  walls,  extending  within  1,200  feet  of  the  sum- 
mit, over  which  come  the  massive  Coal-measure  limestones  forming  the  top 
of  the  peak. 


158  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

The  following  section  gives  the  broader  divisions  of  the  beds  from 
base  to  summit,  including  those  exposed  on  Newark  Mountain,  as  the 
Nevada  limestones  on  Diamond  Peak  are  shown  only  to  a  very  limited 
extent: 

Feet. 
1.  Bluish  gray  distinctly  bedded  limestones 1, 000 


g 

2.  Green  and  brown  and  chocolate  colored  clay  shales,  with  interbedded 

siliceous  bands  and  cherty  beds 500 

„   ~\    O     T\, — 

I 

o 


i 
I 


3.  Dark  gray  quartzites,  compact  conglomerates,  with  interbedded  layers 
of  jasper  and  siliceous  grits.  Near  the  base  narrow  belts  of  blue 
limestone,  carrying  Products  semireticulatug 2, 500 

'4.  Black  argillaceous  shale,  more   or  less  arenaceous  and  similar  to  the 

lower  black  shale 1, 000 

5.  Compact,    fine  grained  sandstone,  with  minute    dark  siliceous  pebbles 

scattered  through  the  beds 100 

6.  Black   argillaceous  shale,  with  fine  intercalated    beds  of  arenaceous 

shale.    These  shales  crumble  on  exposure  to  atmospheric  influence. .      500 

7.  Reddish  gray  shaly  calcareous  beds  . .    100 

8.  Dark  gray  heavily  bedded  siliceous  limestone,  passing  into  bluish  gray 

limestone,  in  places  finely  banded 3, 500 

Total  9, 200 

The  importance  of  this  section  lies  in  the  fact  that  it  gives  over  9,000 
feet  of  conformable  limestones,  shales,  and  sandstones  of  Upper  Devonian 
and  Lower  Coal-measure  strata,  the  best  section  as  yet  recorded  from 
this  portion  of  the  Paleozoic  series  in  Nevada.  It  will  be  noticed  that  at 
the  base  of  this  series  of  beds  less  than  one-half  of  the  thickness  of  the 
Nevada  limestone  is  represented,  and  at  the  top  only  about  one-quarter  of 
the  entire  thickness  assigned  to  the  Lower  Coal-measures  is  exposed  on  the 
summit  of  Diamond  Peak. 

Along  the  summit  of  the  range  occupying  the  axis  of  the  fold  the 
Coal-measure  limestone  extends  for  a  long  distance,  and  on  Diamond  Table, 
at  their  southern  limit,  they  present  a  bold  body  of  nearly  horizontal  beds, 
300  feet  in  thickness,  resting  directly  upon  the  quartzites.  In  Water 
Canyon,  which  drains  the  southern  end  of  Diamond  Peak,  the  position  of 
these  two  formations  is  well  brought  out,  erosion  having  carved  a  mag- 
nificent amphitheater,  with  abrupt  walls,  2,000  feet  into  the  quartzite.  In 
the  bottom  of  the  canyon  the  White  Pine  shale  comes  out  beneath  the 
quartzites,  all  three  formations  being  shown  in  the  canyon  walls. 

Scattered  throughout  these  limestones    may  be  found  Coal-measure 


ALPHA  AND  FUSILINA   PEAKS.  159 

fossils,  the  best  locality  noticed  being  on  the  summit  of  the  ridge  about 
one-third  of  a  mile  south  of  the  peak  and  150  feet  below  the  highest  point. 
Ten  species  were  obtained  here,  the  list  being  given  on  page  91.  The  two 
most  interesting  species  are  Spirifera  trigonalis  and  Camarophoria  cooperensis, 
the  latter  identical  with  the  Missouri  form.  Both  of  them,  as  pointed  out 
by  Mr.  Walcott,  are  characteristic  of  the  lower  Carboniferous  in  the  Mis- 
sissippi Valley.  It  is  these  two  species  that  serve  to  correlate  the  low  lime- 
stone ridges  south  of  Newark  Mountain  with  the  base  of  the  Lower  Coal- 
measures. 

Immediately  northwest  of  the  crest  of  the  ridge  the  strata  dip  easterly, 
and  at  about  the  same  distance  below  the  summit,  as  observed  on  the 
opposite  side  of  the  peak,  the  quartzites  come  in  conformably  beneath  the 
limestones,  dipping  easterly  into  the  ridge.  No  considerable  thicknesses  of 
quartzites  are  exposed,  as  they  are  abruptly  cut  off  by  the  profound  Alpha 
Peak  fault,  which  brings  the  Upper  Coal-measure  limestones  unconform- 
ably  against  them.  Following  the  quartzites  southward,  they  are  seen  to 
be  much  broken  up  and  dislocated,  and  southwest  of  the  peak  again  dip 
westerly,  with  an  angle  of  about  15°,  a  dip  which  they  maintain  as  far 
south  as  Bold  Bluff,  where  they  terminate  abruptly  against  the  Newark 
fault.  By  reference  to  atlas  sheet  vi  the  position  of  the  quartzites  may  be 
readily  made  out,  completely  encircling  Diamond  Peak  on  all  sides. 

Newark  Fauit.-This  line  of  faulting,  starting  in  at  Bold  Bluff,  trends 
southward  along  the  abrupt  west  wall  of  Hayes  Canyon,  following  the 
contact  between  the  two  dissimilar  formations — the  gray  Lower  Coal- 
measures  and  the  black  White  Pine  shale.  It  is  easily  traceable  for  nearly 
3  miles.  At  the  southern  end  it  gradually  trends  off  to  the  southeast,  com- 
pletely cutting  off  the  shales,  as  well  as  the  Diamond  Peak  quartzite,  and 
at  the  mouth  of  Hayes  Canyon  brings  the  Lower  Coal-measures  directly 
against  the  Nevada  limestone  of  Newark  Mountain. 

Region  of  Alpha  and  Fusiiina  Peaks.— The  Lower  Coal-measure  limestone 
overlying  the  Diamond  Peak  quartzite  forms  an  unbroken  narrow  ridge, 
extending  southward  for  over  9  miles,  and  falling  away  gradually  until 
it  passes  beneath  the  Quaternary  of  the  valley.  This  ridge  presents  great 
simplicity  of  structure  and  monotony  of  appearance,  the  beds  exhibit- 


160  v  GEOLOGY  OF  THE  EUKEKA  DISTRICT. 

ing  much  the  same  lithological  habit  throughout  and  everywhere  lying 
inclined  toward  the  west  at  high  angles. 

At  Bold  Bluff,  where  the  quartzite  gives  out,  the  Newark  fault  brings 
the  lower  members  of  the  limestone  next  the  White  Pine  shale.  Along  the 
west  side  of  Hayes  Canyon  both  formations  dip  into  the  ridge,  but  it  is 
somewhat  difficult  to  recognize  the  unconformity  along  the  contact,  owing 
to  the  amount  of  debris,  in  spite  of  the  fact  that  the  angle  of  dip  between 
the  two  horizons  varies  from  20°  to  30°.  Several  observations,  taken  at 
different  points  along  the  canyon  wall,  gave  about  25°  as  the  angle  of 
unconformity.  The  evidence  of  the  unconformity  is  strengthened  by  the 
absence  of  the  entire  thickness  of  quartzite,  the  true  position  of  which, 
between  the  limestone  and  shale,  is  so  well  exhibited  both  on  the  east  side 
of  Diamond  Peak  and  in  the  neighborhood  of  Bold  Bluff  and  Water 
Canyon.  Again,  the  wedging  out  of  the  White  Pine  shale,  which  is 
completely  lost  at  the  mouth  of  Hayes  Canyon,  gives  additional  evidence  of 
the  unconformity. 

The  upper  members  of  the  Lower  Coal-measures  are  quite  as  sharply 
defined  on  the  west  side  by  the  Alpha  fault,  which  for  a  short  distance 
follows  along  the  steep  northwest  slope  of  Diamond  Peak,  bringing  the 
Upper  Coal-measures  unconformably  against  the  quartzite.  Nearly  due  west 
of  the  summit  the  fault  trends  off  to  the  southwest  and  the  Lower  Coal- 
measures  come  in  next  the  quartzite,  the  line  of  fault  marking  the  contact 
between  the  two  bodies  of  Carboniferous  limestone.  The  Alpha  fault  con- 
tinues southward  along  the  base  of  Alpha  Peak,  but  terminates  abruptly 
on  reaching  the  north  slope  of  Weber  Peak.  It  is  rarely  that  an  uncon- 
formity in  Carboniferous  limestone  strata  is  more  strikingly  shown  than  by 
the  two  Coal-measure  formations  along  the  Alpha  fault.  There  may  be 
seen  here  on  one  side  of  the  fault,  the  underlying  limestones  dipping  west- 
ward at  angles  varying  from  65°  to  85°,  and  on  the  opposite  side,  the  over- 
lying limestones  inclined  at  angles  rarely  exceeding  10°. 

At  Weber  Peak,  where  the  Alpha  fault  terminates,  an  east  and  west 
fault  brings  up  the  Weber  conglomerate,  and  from  here  southward  the  beds 
of  the  latter  epoch  are  found  in  their  true  geological  position  conformably 
overlying  the  Lower  Coal-measures.  This  east  and  west  fault  does  not 


WEBER  PEAK.  1(}1 

cross  the  Alpha  fault,  at  least  the  limestones  appear  to  have  undergone  no 
displacement,  West  of  the  Alpha  displacement  the  course  of  the  east  and 
west  fault  after  passing  Weber  Peak  is  lost,  being  buried  beneath  the  accu- 
mulations of  igneous  rocks. 

The  thickness  of  the  Lower  Coal-measures  may  be  best  estimated 
south  of  Fusilina  Peak,  where  the  upper  members  of  the  epoch  are  deter- 
mined by  the  position  of  the  Weber  conglomerate,  and,  although  there  ex- 
ists no  positive  evidence  that  the  beds  resting  on  the  White  Pine  shale  are 
the  equivalent  of  the  lowest  members  found  elsewhere,  they  probably  do 
not  belong  far  above  the  base.  It  is  estimated  that  the  limestones  measure 
about  3,800  feet  in  thickness. 

Organic  remains  may  be  found  scattered  throughout  the  limestone,  but 
nowhere  were  any  grouping  of  species  obtained  which  were  of  special  in- 
terest or  which  could  be  regarded  as  the  equivalent  of  the  Spring  Hill 
fauna.  At  the  head  of  Newark  Canyon,  which  starts  in  near  the  base  of 
the  limestone  immediately  resting  on  the  White  Pine  shale,  were  found 
Producing  longispinwSj  P.  semireticulatus,  and  Spirifera  camerata,  while  south  of 
Fusilina  Peak,  at  the  top  of  the  horizon,  the  same  species  occur  associate!  1 
with  Fusilina  cylindrical  and  other  Coal-measure  forms.  On  the  map  will  be 
found  a  number  of  localities  designated  where  fossils  were  procured  but 
they  indicate  only  a  few  of  the  horizons  where  they  are  known  to  exist. 

Weber  Peak  and  Pinto  Springs  Region.— Under  this  heading  may  be  designated 
the  area  of  the  Weber  conglomerates  lying  between  the  two  great  bodies 
of  Carboniferous  limestone.  From  Weber  Peak  southward  they  overlie 
conformably  the  Lower  Coal-measures,  although  not  extending  southward 
out  into  the  open  valley  quite  as  far  as  the  limestone,  being  buried  beneath 
either  basaltic  flows  or  the  alluvial  deposits  of  Pinto  Creek.  Along  the 
line  of  contact  the  Weber  conglomerates  form  a  well  defined  series  of  ridges 
parallel  with  the  Alpha  and  Fusilina  ridges,  the  two  formations  standing  out 
sharply  contrasted  by  their  surface  forms,  atmospheric  agencies  acting  quite 
differently  on  the  fine  crystalline  limestones  and  the  coarse  conglomerates. 
In  like  manner  erosion  acting  upon  the  more  easily  disintegrated  conglom- 
erates has  worn  out  a  number  of  narrow  drainage  channels  along  the  con- 
tact which  serve  still  more  sharply  to  define  tin-  boundaries.  The  conglom- 
MON  xx 11 


162  GEOLOGY  OF  THE  EUKEKA  DISTRICT. 

erates  stretch  out  toward  the  west  until  cut  off  by  the  broad  basaltic  table- 
land of  the  Strahleuberg,  which,  concealing-  everything  over  a  wide  area, 
leaves  to  conjecture  the  probable  structural  relations  of  the  Carboniferous 
rocks  of  the  Diamond  Range  to  the  immense  block  of  Devonian  limestone 
of  the  County  Peak  uplift.  East  of  Strahlenberg,  the  highest  point  on  the 
eastern  rim  of  the  basaltic  field,  the  conglomerates  present  a  broad,  high 
ridge,  with  a  strike  of  N.  30°  W.  and  an  easterly  dip  of  75°.  It  is  against 
this  ridge  that  the  basalts  have  been  piled  up,  the  ridge  acting  as  a  barrier 
to  their  further  progress  in  that  direction.  Between  the  basalt  and  the 
Lower  Coal-measures  of  Alpha  Ridge  the  conglomerates  are  plicated  into 
a  broad  syncline  followed  by  a  sharp  anticline,  the  axes  of  both  folds  being 
traceable  the  entire  length  of  the  conglomerate  area.  The  conglomerate 
ridge  lying  next  to  the  Lower  Coal-measures  forms  the  east  side  of  the 
syncline,  the  beds  coming  up  again  on  the  opposite  side  of  the  fold  in  a 
ridge  nearly  parallel  with  the  first  one.  The  anticlinal  fold  presents  a  much 
sharper  axis,  the  beds  011  both  sides  of  the  arch  dipping  at  angles  varying 
from  55°  to  65°. 

One  of  the  most  fortunate  occurrences  in  working  out  the  structural 
geology  of  the  region  is  the  belt  of  light  gray  Upper  Coal-measure  lime- 
stone lying  between  the  westerly  dipping  beds  of  the  anticlinal  fold  and 
the  basalts.  It  furnishes  within  the  district  evidence  of  the  position  of 
the  Weber  conglomerate  between  the  two  great  belts  of  Coal-measure  lime- 
stone and  although  ample  proof  could  be  found  elsewhere,  it  makes  the 
chain  of  evidence  complete  for  all  the  divisions  of  the  Paleozoic  series 
of  rocks  in  the  Great  Basin.  It  is  a  narrow  strip  of  limestone,  in  places 
only  a  few  hundred  feet  in  width  and  about  one  mile  in  length,  being  cut 
off  both  at  the  north  and  south  by  igneous  rocks.  It  strikes  nearly  north 
and  south  and  dips  between  55°  and  60°  to  the  west,  coinciding  with  the 
inclination  of  the  underlying  conglomerates  on  the  west  side  of  the  anti- 
clinal fold.  In  a  yellowish  gray  bed  occurs  a  characteristic  fauna  of  the 
Upper  Coal-measures ;  a  list  of  the  species  procured  here  will  be  found 
elsewhere.  The  continuity  of  this  body  of  Upper  Coal-measure  limestone 
with  the  larger  body  north  of  Newark  Canyon  is  broken  not  only  by 
igneous  flows,  but  the  connection  is  completely  severed  by  a  line  of  fault- 


WEST  SLOPE  OF  DIAMOND  RANGE.  1C,.; 

ing  along  the  canyon.  The  distance  between  them  measures  only  about 
one-half  mile  and  is  mainly  occupied  on  the  surface  by  rhyolitic  pumices 
and  tuffs. 

No  special  mention  need  be  made  of  the  physical  characters  of  the 
Weber  conglomerate,  as  it  has  been  described  in  sufficient  detail  in  the  chap- 
ter devoted  to  the  Carboniferous  rocks,  nearly  all  the  observations  there 
given  being  taken  from  this  region. 

west  slope  of  Diamond  Range.— From  Newark  Canyon  northward  and  west- 
ward of  the  Alpha  fault,  the  country,  both  in  topographical  features  and 
geological  structure,  presents  much  the  same  general  aspect  over  the  entire 
area.  It  is  the  most  monotonous  and  least  disturbed  region  within  the 
limits  of  the  survey.  The  opposite  sides  of  Newark  Canyon  offer  marked 
geological  contrasts;  on  the  one  side  folded  and  distorted  beds  of  coarse 
conglomerates,  on  the  other  a  uniformly  inclined  slope  of  limestones.  Along 
the  lower  end  of  the  canyon  the  contact  of  the  two  rocks  is  broken  by 
overflows  of  pumices,  tuffs,  and  basalts,  but  higher  up  and  north  of  the 
drainage  channel  the  relations  between  the  two  horizons  are  strikingly  shown 
on  the  north  slope  of  Weber  Peak  about  150  feet  below  the  summit.  Here 
the  conglomerates  lie  inclined  about  18°  to  the  west,  with  the  limestones 
resting  against  them  at  an  angle  of  only  6°,  but  without  any  essential 
difference  in  their  strike,  both  rocks  following  the  trend  of  the  Alpha  and 
Fusiliua  ridge.  This  change  is  all  the  more  strongly  marked  by  the  con- 
trast in  topographical  features  and  unconformity  of  strata  between  the  two 
bodies  of  limestone  on  the  opposite  sides  of  the  Alpha  fault.  This  region 
is  sharply  denned  by  the  Alpha  fault  on  the  east.  From  the  fault  to  the 
Quaternary  deposits  of  Diamond  Valley  there  is  a  nearly  uniform  slope 
three  miles  in  width,  with  a  fall  of  over  1,200  feet.  It  is  crossed  by  fre- 
quent drainage  channels  at  fairly  regular  intervals,  all  of  them  having  a 
course  a  little  north  of  west.  Nowhere  have  they  cut  down  into  the  under- 
lying limestones  more  than  a  few  hundred  feet,  the  bottoms  of  the  valley-, 
as  a  rule,  being  shallow  ravines  with  narrow  strips  of  meadow  land  along 
the  stream  bottoms.  All  the  intervening  slopes  present  much  the  same 
superficial  features,  for  the  most  part  smoothly  worn  down,  with  here  and 


164  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

there  an  occasional  elevation,  seldom  rising  more  than  100  feet  above  the 
average  height  of  the  surrounding  country. 

Over  this  entire  area  the  only  rocks  which  have  been  recognized  are 
the  Upper  Coal-measures,  inclined  toward  the  west  at  low  angles  agreeing 
closely  with  the  slope  of  the  country.  This  prevents  any  considerable 
thickness  of  strata  being  exposed,  and  it  is  doubtful  if  there  can  be  seen 
here  a  greater  development  of  beds  than  those  found  south  of  Newark 
Canyon,  where  it  is  estimated  that  500  feet  are  shown  in  the  ridge 
which  rises  above  the  basaltic  flows.  At  the  latter  locality  the  base  of  the 
horizon  is  unquestionably  exposed,  but  along  the  line  of  the  Alpha  fault 
there  is  no  structural  evidence  that  the  basal  rocks  come  to  the  surface. 
Almost  anywhere"  scattered  through  these  limestones  organic  remains 
characteristic  of  the  Coal-measures  may  be  found,  but  the  most  promising 
field  for  collection  is  on  the  summit  of  the  ridge  just  north  of  Garden 
Canyon.  Nearly  all  the  forms  obtained  here  are  common  enough  elsewhere, 
with  the  exception  of  Ptilodictya  (Stenopera)  carlonaria  and  P.  serrata.  Far 
to  the  south  of  this  latter  locality,  north  of  Weber  Peak,  and  just  above  the 
Alpha  fault,  occurs  a  nearly  similar  grouping  without  the  latter  two  forms, 
but  with  the  addition  of  Macrodon  tenuistruita. 

Perhaps  the  most  important  geological  feature  of  this  inclined  table  of 
Upper  Coal-measure  limestone  is  the  occurrence  of  an  interstratified  bed 
of  conglomerate  varying  in  thickness  from  15  to  20  feet.  It  is  exposed  in  one 
or  two  of  the  long  ridges  stretching  out  toward  Diamond  Valley,  and  in  one 
instance  occupies  a  low  depression  on  the  top  of  the  ridge.  This  conglomer- 
ate is  made  up  of  pebbles  of  chert,  jasper  and  quartz  such  as  are  found 
throughout  the  Weber  epoch,  firmly  cemented  together  into  a  hard  sand- 
stone. Mingled  with  these  siliceous  pebbles  occur  rounded  fragments  of 
limestone  carrying  organic  remains  such  a&Syringopora  and  Fttyilhta  cylindrica 
and  other  forms  common  to  the  Carboniferous  limestones  below  the  Weber 
conglomerate,  but  in  no  instance  are  specific  forms  obtained  other  than  those 
previously  recognized  in  the  underlying  limestones.  This  implies  that  after 
the  deposition  of  the  lower  portion  of  the  Upper  Coal-measures  the  country 
underwent  some  slight  changes  in  elevation,  subjecting  the  Weber  con- 
glomerate and  Lower  Coal-measures  to  the  influences  of  erosion,  the  mate- 


NEW  YOEK  HILL.  H;;> 

rial  being  redeposited.  All  indications  point  to  the  fact  that  this  material  of 
the  interbedded  conglomerates,  was  derived  from  some  land  mass  in  close 
proximity  to  the  present  beds,  as  it  seems  hardly  possible  from  the  size  and 
nature  of  the  easily  disintegrated  limestone  that  it  could  have  been  exposed 
for  any  great  length  of  time  to  subaqueous  influences. 

CARBON   RIDGE   AND    SPRING   HILL    GROUP. 

The  area  embraced  within  this  block  is  situated  in  the  center  of  the 
Eureka  Mountains  and  stretches  in  a  narrow  belt  from  Diamond  Valley  to 
Fish  Creek  basin.  It  lies  hemmed  in  between  Prospect  Ridge  and  the 
County  Peak  and  Silverado  uplift,  presenting  somewhat  the  appearance  of  a 
depressed  and  broken  region  bounded  by  two  elevated  and  well  defined 
mountain  masses.  This  appearance  is,  in  part,  due  to  its  relatively  slight 
elevation,  and  in  part  to  the  struggle  for  supremacy  between  sedimentary 
strata  and  the  volcanic  lavas  spread  out  over  them  concealing  large  areas 
and  breaking  the  continuity  of  strata.  At  Pinto  Peak  the  rhyolites  have 
been  piled  up  until  they  have  attained  an  elevation  higher  than  any  point 
reached  by  the  upturned  limestones.  These  igneous  rocks  divide  the  sedi- 
mentary beds  into  two  areas,  one  a  northern,  of  which  Spring  Hill  is  the 
center,  the  other  to  the  south,  designated  as  Carbon  Ridge.  Both  regions, 
however,  present  much  the  same  geological  conditions  and  consist  wholly 
of  Carboniferous  rocks,  the  only  two  epochs  represented  being  the  Lower 
Coal-measures  and  Weber  conglomerate. 

New  York  Hill.— The  direct  contact  between  the  Silurian  and  Carbonifer- 
ous rocks  on  opposite  sides  of  the  Hoosac  fault  may  be  best  seen  where  the 
Lower  Coal-measures  of  New  York  Hill  rest  against  the  Lone  Mountain 
limestones  of  McCoy's  Ridge,  as  along  the  fault  between  these  two  ridges 
no  lavas  have  reached  the  surface  to  obscure. the  sedimentary  beds.  New 
York  Hill  is  in  some  measure  isolated  from  the  rest  of  the  Carboniferous 
rocks,  being  completely  surrounded  by  lines  of  faulting.  On  two  sides  the 
Hoosac  fault  outlines  it  from  the  Prospect  Ridge  uplift  while  a  secondary 
fault  of  but  slight  displacement  breaks  the  continuity  of  strata  between  the 
hill  and  the  beds  underlying  Richmond  Mountain  on  the  east  and  Spring 
Hill  on  the  south.  The  limestones  of  New  York  Hill  strike  approxi- 


166  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

mately  parallel  with  the  trend  of  the  canyon,  which  in  turn  coincides  with 
that  of  the  Hoosac  fault.  The  beds  dip  uniformly  to  the  southeast  with  an 
average  inclination  of  30°.  There  is  no  direct  means  of  determining  the 
base  of  the  Lower  Coal-measures  anywhere  in  the  Spring  Hill  block, 
although  the  lower  beds  of  New  York  Hill  are  probably  not  far  from  the 
base  of  the  epoch  and  occur  as  low  down  in  the  series  as  any  strata  to  be 
found  along  the  east  side  of  the  Hoosac  fault.  Between  the  base  of  the 
Lower  Coal-measures  at  Diamond  Peak  and  those  of  New  York  Hill  some 
resemblance  may  be  traced,  but  lithological  evidence  is  not  of  much 
value,  as  the  beds  change  rapidly  in  the  character  of  their  sedimentation. 
On  the  west  slope  of  New  York  Hill,  Coal-measure  fossils  may  be  found 
scattered  through  the  beds  and  in  one  locality  in  a  shaly  limestone  near 
the  summit  the  following  species  were  collected : 

Fusilina  cylindrica.  Productus  nebrascensis. 

Fusilina  robusta.  Productus  prattenianus. 

Productus  semireticulatus. 

At  the  extreme  northeast  end  of  New  York  Hill  the  drainage  channel, 
instead  of  following  closely  the  line  of  the  fault  between  the  Silurian  and 
Carboniferous  rocks,  deviates  to  the  northward,  cutting  through,  for  some 
unexplained  reason,  the  Lone  Mountain  strata,  leaving  a  portion  of  the 
latter  limestone  resting  upon  the  slope  of  New  York  Hill  on  the  south  side 
of  the  canyon.  At  the  northeast  end  of  New  York  Hill,  but  east  of  the 
Silurian  limestone,  occurs  100  feet  or  more  of  thinly  bedded  clays,  grits, 
and  argillaceous  limestones,  passing  into  purer  beds,  which  are  apparently 
unconformable  with  the  main  body  of  limestones  as  they  dip  to  the  north- 
ward, toward  the  fault,  at  an  angle  of  30°.  They  occupy  only  a  small  area, 
but  it  seems  difficult  to  tell  just  how  they  are  related  to  the  main  body  of 
limestone,  or  to  connect  them  in  the  section  with  the  southeasterly  dipping 
beds.  That  they  are  low  down  in  the  limestones  is  evident  from  the  fact 
that  they  can  be  con-elated  with  the  beds  on  the  east  side  of  Eureka  Can- 
yon which  lie  near  the  base  of  the  uplifted  strata,  dipping  under  Richmond 
Mountain.  Their  geological  position  would  be  of  no  importance  except 
that  it  is  in  these  beds  that  the  fresh  and  brackish  water  shells  occur  which 
have  already  been  described  in  the  chapter  devoted  to  the  Carboniferous 


FRESH  WATEE  FAUNA.  167 

rocks.  Their  mode  of  occurrence  everywhere  shows  evidence  of  shallow 
water,  but  they  rapidly  pass  into  beds  indicating  much  deeper  water. 
Associated  with  these  fresh-water  shells  are  numerous  fragments  of  plant 
remains,  proving,  without  doubt,  the  existence  at  no  great  distance  of  a 
land  surface.  The  specific  characters  of  these  shells  will  be  found  described 
elsewhere  by  Mr.  Walcott.1 

Limestone  of  Richmond  Mountain.— Between  Eureka  Canyon  and  Richmond 
Mountain  lies  a  body  of  limestone  uniformly  inclined  to  the  east  until  it 
passes  beneath  the  andesites  of  the  latter  mountain.  It  rises  nearly  700 
feet  above  the  valley,  with  a  fairly  regular  slope,  except  where  trenched  by 
short  drainage  channels  which  have  cut  deeply  into  the  rock,  giving  the 
ridge  a  somewhat  ribbed  appearance.  The  beds  strike  N.  16°  E.  and 
dip  from  40°  to  50°  under  the  lavas.  The  Richmond  Smelting  Works  are 
situated  near  the  northern  end  of  this  limestone  body.  Just  back  of  the 
smelters  the  base  of  the  limestones  are  well  exposed,  and  near  the  rail- 
way cut  there  may  be  seen  a  good  exposure  of  strata.  At  the  base  of 
the  cliff  occurs  a  series  of  dark  argillaceous  shales  of  unknown  thickness 
weathering  011  exposure  to  blue  and  gray  clays.  In  these  clays  may  be 
found  indications  of  plant  remains  associated  with  the  Physa  prisca  and 
Ampularia  obtained  on  the  opposite  side  of  the  ravine  in  New  York  Hill, 
the  shells  serving  to  correlate  the  beds.  It  is  to  be  regretted  that  their 
strike  and  dip  could  not  be  determined  with  precision,  but  they  give  every 
appearance  of  passing  conformably  beneath  the  overlying  strata. 

The  following  section  was  made  across  the  strata  extending  from  the 
top  of  the  series  down  to  the  clay  beds  at  the  base: 

Feet. 

1.  Coarse  conglomerate  cemented  in  fine  arenaceous  grains 50 

2.  Compact  gray  and  yellow  sandstones  carrying  a  little  calcareous  material, 

and  occasional  thin  belts  of  limestone 200 

3.  Fine  smooth  pebbles  in  a  yellow  matrix 100 

4.  Brownish  white  sandstone 

5.  Fine  conglomerate,  with  an  admixture  of  calcareous  material  throughout-  100 

6.  Gray  limestone,  passing  into  a  light  gray  and  yellowish  sandstone  .  75 

7.  Cherty  limestone,  passing  into  fine  siliceous  limestone 73 

8.  Light  colored  and  banded  vitreous  quartzite 25 

1  Paleontology  of  the  Kureka  District,  Mon.  vin,  U.  S.  Geolo>tH'»l  Survey,  p.  261. 


168  GEOLOGY  OF  THE  EUEEKA  DISTRICT. 


Feet. 


9.  Cherty  bluish  gray  limestone,  carrying  Griffithides  portlocki,  Productus 

semireticulatus,  P.  lonffispinvs,  P.  prattenianus,  Fusilina  cylindrica. . .          300 

10.  Blue  limestones  in  massive  layers,  with  thin  iuterbedded  calcareous  shales 

carrying  Pleurotomaria  conoidea,   Metoptomia    peroccidens,   Macro- 
cheilus,  Nucula,  Ortkocefas,  Leperditia 400 

11.  Dark  argillaceous  shales,  weathering  to  blue  and  gray  clays,  carrying 

fresh  water  shells  and  plant  remains Unknown  thickness. 

1,525 

Throughout  the  entire  series  of  beds  above  the  quartzite  band  (No.  8) 
occurs  a  grouping-  of  characteristic  Coal-measure  fossils  from  which 
twenty-eight  species  have  been  determined.  The  list  will  be  found  in  the 
chapter  devoted  to  the  Carboniferous  rocks.  Overlying  the  limestones  the 
andesitic  rocks  rise  in  precipitous  walls  for  over  800  feet. 

Spring  Hiii.— The  uppermost  members  of  the  Richmond  Mountain  beds 
are  traceable  across  Eureka  Canyon,  the  conglomerates  standing  out 
conspicuously  along  the  west  slope  of  Spring  Hill  dipping  into  the 
ridge.  A  line  of  displacement  runs  along  the  Secret  Canyon  Road  valley, 
and,  as  it  approaches  the  Hoosac  fault,  the  continuity  of  strata  becomes 
more  and  more  difficult  to  follow,  showing  signs  of  displacement  under  the 
influence  of  the  outpouring  of  lavas  near  the  centers  of  volcanic  activity 
About  a  mile  up  the  valley  a  complete  change  in  structure  takes  place  and 
a  low  hill,  somewhat  isolated  from  the  ridge,  stands  out  between  the  main 
body  of  Spring  Hill  and  the  Hoosac  fault.  It  rises  about  400  feet 
above  the  level  of  Secret  Canyon  Road  and  from  its  peculiar  outlines,  the 
result  of  erosion,  it  has  been  designated  as  Conical  Hill.  It  presents  a 
small  block  of  Lower  Coal-measure  strata  which,  instead  of  dipping  easterly 
in  conformity  with  the  rest  of  Spring  Hill,  forms  an  anticline  with 
the  main  ridge,  the  beds  dipping  westerly  directly  toward  the  Hoosac 
fault.  On  Conical  Hill  the  strata  strike  from  N.  20°-25°  E.,  parallel  with 
the  Canyon  Road  valley,  and  dip  30°  W.1  On  both  sides  of  the  axis  of 
the  fold  the  series  of  beds  are  easily  traced,  consisting  of  limestones,  cal- 
careous shales,  arenaceous  layers,  with  a  well  defined  bed  of  coarse  con- 
glomerate about  75  feet  in  thickness.  This  conglomerate  appears  on  the 

1  Owing  to  an  error  in  the  proof-reading  of  tho  map,  the  beds  on  Conical  Hill  are  represented  as 
inclined  steeply  to  the  east,  whereas  the  dip  of  30°  to  the  west,  as  given  in  tho  text,  is  correct. 


SPRING  HILL.  169 

west  side  of  Conical  Hill  and  again  near  the  summit  of  Spring  Hill, 
standing  out  prominently  on  both  sides  of  the  fault  as  a  well  denned  body, 
serving  as  an  excellent  datum  ledge  in  determining  the  position  of  the  beds. 
The  transition  from  the  calcareous  to  the  siliceous  beds  is  rapid,  both  above 
and  below  the  conglomerate.  This  description  of  Conical  Hill  is  given 
somewhat  in  detail,  as  it  is  here  that  the  Lamellibranchiate  fauna  of  the 
Carboniferous  occurs.  On  the  east  slope  of  this  hill,  near  the  saddle  which 
connects  it  with  Spring  Hill,  there  is  found  in  a  shaly  limestone  a  small 
but  most  typical  Coal-measure  fauna.  Above  these  shaly  beds,  about  200 
or  300  feet,  occur  the  limestones  carrying  the  Lamellibranchiate  fauna,  asso- 
ciated with  Coal-measure  species,  as  described  in  the  chapter  on  Carbon- 
iferous rocks.  Ovei  lying  the  Lamellibranchiate  beds,  on  the  east  side  of 
the  fold,  on  the  east  side  of  Spring  Hill,  characteristic  Coal-measure  fossils 
come  in,  but  without  the  mingling  of  the  fauna  found  below. 

These  limestones  are  in  turn  overlain  by  a  belt  of  fine  conglomerate 
100  feet  in  thickness,  in  places  altered  to  an  indurated  sandstone,  which 
forms  the  lower  slope  of  the  ridge  on  the  west  side  of  Eureka  Canyon  south 
of  Spring  Hill.  It  crosses  the  canyon  near  the  toll-house,  with  a  strike 
of  N.  16°  E.  and  is  traceable  on  the  opposite  hills  without  difficulty.  At 
the  east  base  of  Spring  Hill,  along  the  bottom  of  the  Eureka  Canyon 
and  underlying  these  conglomerates,  occurs  a  thin  band  of  black,  fissile, 
argillaceous  shale,  from  which  were  collected  Spirifera  lineata  and  a  small 
Discina  not  unlike  D.  minuta.  This  shale  varies  somewhat  in  thickness,  but 
was  estimated  at  50  feet.  The  origin  of  the  canyon  is  in  part  due  to  a 
fracture  in  the  quartzite  and  in  part  to  the  nature  of  the  easily  eroded 
shales,  but  it  does  not  appear  to  be  accompanied  by  any  considerable 
amount  of  displacement  of  strata,  as  is  the  case  with  nearly  all  the  other 
principal  longitudinal  drainage  channels;  in  this  respect,  however,  it  re- 
sembles Secret  Canyon.  Overlying  the  conglomerates  blue  and  gray  lime- 
stones continue  on  up  to  the  summit  of  the  section,  with  occasional  thin 
bands  of  chert  and  arenaceous  layers,  but  with  less  and  less  siliceous  mate- 
rial. On  the  top  of  the  ridge  east  of  the  toll-house  the  gray  limestones 
carry  a  typical  Coal-measure  fauna,  and  in  a  thin  bed  on  the  west  side, 
about  100  feet  below  the  summit,  there  were  collected: 


170  GEOLOGY  OF  THE  EUEEKA  DISTEICT. 

Fusilina  cylindrica.  Productus  longispinus. 

Chonetes  verneuiliana.  Productus  punctatus. 

Productus  costatus.  Productus  semireticulatus. 

Spring  Hill  and  the  limestone  ridge  lying  on  the  west  side  of  the  Pinto 
fault  form  a  synclinal  fold  whose  axis  is  situated  on  the  western  side  of  the 
high  hill  east  of  the  toll  road.  The  strata  dip  away  from  the  Pinto  fault 
into  the  ridge  at  high  angles,  but  on  the  opposite  side  of  the  fold  they 
lie  more  regularly  inclined  at  a  much  lower  angle.  The  synclinal  structure 
here  does  not  differ  essentially  from  that  shown  southward  along  the 
geological  section  E-F,  atlas  sheet  xui. 

On  the  south  side  of  Conical  Hill  a  fault  coincides  with  a  narrow  ravine 
separating  it  from  the  next  hill  to  the  south.  Both  the  ravine  and  fault 
trend  to  the  south  and  the  latter  is  finally  lost  beneath  the  audesites.  On 
this  second  hill  the  beds  are  still  in  accord  with  those  of  Conical  Hill  and 
dip  westerly,  but  to  the  southward  of  it  runs  a  cross  fault  connecting  the 
Hoosac  fault  with  the  Conical  Hill  fault.  To  the  south  of  this  cross  fault 
the  limestones  again  dip  easterly  in  conformity  with  those  of  Spring  Hill. 

A  short  distance  south  of  this  latter  fault  the  geological  section  E-F, 
atlas  sheet  xni,  crosses  the  Carboniferous  rocks  lying  between  the  Hoosac 
and  Pinto  faults.  The  entire  block  of  limestones  west  of  the  Conical  Hill 
fault  dips  easterly  at  about  30°.  With  apparently  only  a  slight  break  in 
the  strata  along  this  displacement  the  beds  on  the  east  side  of  the  fault-plane 
still  dip  easterly  at  about  the  same  angle  followed  by  a  synclinal  fold,  the 
westerly  beds  of  which  attain  angles  as  high  as  70°  and  both  north  and 
south  of  the  cross-section  reaching  even  80°.  Taken  as  a  whole,  the  Car- 
boniferous rocks  included  within  this  block  consist  of  limestone  strata  more 
or  less  arenaceous  with  interstratified  belts  of  both  fine  and  coarse  con- 
glomerate and  carrying  from  base  to  summit  characteristic  Coal-measure 
species.  It  is  estimated  that  the  Lower  Coal-measure  beds  along  the  line 
of  this  section  have  a  thickness  of  about  3,400  feet,  but  it  is  evident  that 
the  base  of  the  series  is  not  reached,  and  that  there  are  at  least  300  or  400 
feet  of  beds,  and  probably  more,  on  New  York  Hill  and  Richmond  Moun- 
tain unrepresented  here.  Measurements  of  the  Lower  Coal-measures  in  the 
Diamond  Range  calculated  from  observed  strikes  and  dips  give  3,700  feet 


LOWEE  COAL  MEASURE  FAUNA.  171 

of  beds.  From  this  data  the  development  of  the  Lower  Coal-measure 
epoch  at  Eureka  is  placed  at  3,800  feet ;  this  thickness  is  probably  rather 
under  than  over  estimated. 

Region  South  of  Spring  Hiii.-Along  the  divide  which  separates  Spring  Hill 
from  Carbon  Ridge  vast  accumulations  of  andesites,  rhyolites,  pumices,  and 
basalts  have  poured  out,  submerging  over  a  large  area  all  sedimentary 
beds.  An  exception  is  found  in  the  broad,  deeply  eroded  basin  just  north 
of  Pinto  Peak,  Carboniferous  rocks  again  coming  to  the  surface. 

Within  this  basin  occurs  several  exposures  of  limestones,  and  on  the 
north  side  there  is  a  short  narrow  ridge  nearly  200  feet  in  height  in  which 
the  beds  are  seen  to  strike  N.  24°  W.  and  dip  steeply  to  the  east.  At  the 
western  end  of  these  exposures  there  occurs  a  well  denned  belt  of  sand- 
stones, beneath  which  crops  out  an  area  of  clay  shales.  The  latter  are  so 
obscured  by  Quaternary  accumulations  that  but  little  could  be  made  out  of 
them.  They  resemble,  however,  similar  shales  to  the  west  of  Carbon  Ridge. 
The  sequence  of  beds  indicates  their  close  relationship  to  those  of  Spring 
Hill,  but  their  geological  age  is  still  more  strongly  shown  by  the  grouping 
of  fossils  obtained  from  the  limestones.  The  complete  list  is  given  here,  as 
it  is  rather  a  characteristic  grouping  of  the  Lower  Coal-measures  of  Eureka 
and  carries  with  it  a  number  of  species  found  elsewhere  in  the  district  at 
both  lower  and  higher  horizons.  The  list  is  as  follows : 

Stromatopora,  sp.  f  Crenipecten  hallanus. 

Zaphrentis,  sp.  ?  Pterinea  pintoensis. 

Syringopora.  Pinna  consiinUis. 

Ptilodictya.  Myalina  subovata. 

Lingula  mytaloides.  Myalina  congeneris. 

Orthis  resupinata.  Modiomorpha?  pintoensis. 

Cbonetes  granulifera.  Sanguinolites  retusus. 

Productus  seniireticulatus.  Microdon  coimatus. 

Productus  prattenianus.  Schizodus  cuneatns. 

Spirifera  camerata.  Schizodus  pintoensis. 

Spirifera  striata.  Belleroplion  inajusoulus. 

Bhynchonella  eurekensis.  Orthooeras  randolphensis. 

Aviculopecten  pintoensis.  Orthoceras,  sp.  ! 

Aviculopecteu  peroccidens.  Leperditia,  sp.  T 

Streblopteria  similis.  Griffithides  portlockL 


172  GEOLOGY  OF  THE  EUEEKA  DISTRICT. 

Carbon  Ridge.— The  area  included  under  this  designation  is  almost  com- 
pletely encircled  by  volcanic  rocks,  and  nowhere  does  it  come  in  direct 
contact  with  sedimentary  beds  of  adjacent  regions.  The  nearest  approach 
to  such  contact  occurs  just  northeast  of  Gray  Fox  Peak,  where  a  body  of 
rhyolite  about  700  feet  in  width  separates  the  Carboniferous  rocks  from  the 
Eureka  quartzite  situated  on  the  west  side  of  the  Hoosac  fault.  If  the 
superficial  detrital  material  along  the  southeastern  slopes  of  Carbon  Ridge 
were  scraped  away  it  seems  highly  probable  that  the  isolation  of  this  block 
would  be  still  more  noticeable,  as  there  is  good  reason  to  believe  that  igne- 
ous rocks  lie  just  beneath  the  surface.  This  is  indicated  by  the  configura- 
tion of  the  drift-covered  hills,  the  superficial  drainage  channels,  and  the 
nature  of  the  detrital  material  itself.  The  exposures  of  the  andesites,  rhy- 
olites,  and  pumices  which  are  shown  in  the  narrow  ravine  draining  the 
southern  slopes  of  Carbon  Ridge  are  portions  of  much  more  extensive 
bodies  bordering  the  southern  end  of  the  mountains.  Not  only  is  the  con- 
tinuity of  sedimentary  beds  destroyed  by  volcanic  overflows,  but  nowhere 
are  the  Carboniferous  rocks  of  Carbon  Ridge  recognized  immediately  along 
the  lines  of  the  two  great  displacements — the  Hoosac  and  Pinto  faults.  On 
both  sides  of  Carbon  Ridge  the  precise  trend  of  these  faults  is  obscured 
by  igneous  rocks,  although  at  several  localities  it  is  possible  that  they 
may  form  only  superficial  layers  over  the  sedimentary  beds.  Carbon 
Ridge  measures  about  2|  miles  in  length,  but  varies  in  width,  owing  to 
irregularities  in  the  volcanic  flows.  Across  its  widest  expansion,  as  seen 
on  the  surface,  it  measures  1J  miles.  Along  the  summit  of  the  ridge  the 
beds  strike  nearly  north  and  south  and  maintain  an  average  dip  of  70°  to 
the  east,  presenting  a  fairly  regular  uplifted  block  of  limestones  and  con- 
glomerates. Along  the  west  base  of  the  ridge  runs  a  baud  of  gray  granular 
sandstone,  beyond  which  to  the  westward  lies  an  area  of  fissile  clay  shales, 
exhibiting  no  good  exposures  and  without  reliable  dips  and  strikes,  as  they 
are  much  broken  up  and  disturbed,  owing  to  their  proximity  to  the  Hoosac 
fault.  Apparently  they  lie  unconformable  with  the  limestones  of  Carbon 
Ridge,  but  their  relationship  with  the  latter  is  by  no  means  satisfactorily 
made  out.  A  line  of  faulting  of  which  little  is  known  cuts  them  off  from 
the  main  body  of  limestones,  the  shales  lying  at  a  much  lower  angle  than 


THICKNESS  OF  WEBEE  CONGLOMERATE.  173 

the  highly  inclined  beds  of  the  ridge.  It  seems  probable  that  they  are 
identical  with  the  shales  observed  underlying  the  limestones  in  the  expos- 
ures north  of  Pinto  Peak.  On  Carbon  Ridge  the  beds  exhibit  much  the 
same  sequence  of  sediments  as  are  found  in  the  Spring  Hill  region, 
the  limestones  being  more  or  less  siliceous  and  carrying  interbedded  con- 
glomerates. On  the  summit  of  the  ridge  there  is  a  considerable  develop- 
ment of  thinly  bedded  calcareous  shales,  in  places  fossiliferous.  Unlike 
this  horizon  at  Spring  Hill,  abundant  structural  evidence  exists  here  to 
show  that  the  uppermost  members  of  the  Lower  Coal-measure  series  are  rep- 
resented, as  the  Weber  conglomerates  overlie  them  conformably.  Between 
the  beds  of  the  two  epochs  a  peculiar  structural  feature  may  be  noticed  in 
the  narrow  ravines  which  have  been  worn  out  by  erosion  along  the  contact 
of  the  limestones  and  conglomerates.  These  ravines,  which  start  in  with 
approximately  north  and  south  trends,  invariably  curve  to  the  east  and  cross 
the  conglomerates  at  right  angles  to  their  strike,  breaking  up  the  formation 
into  individual  blocks,  which  are  united  to  the  main  body  of  Carbon  Ridge 
by  low,  connecting  saddles  of  conglomerate. 

Everywhere  the  conglomerate  is  seen  to  overlie  the  limestone  conform- 
ably. Estimating  from  the  observed  dips  and  strikes,  the  Lower  Coal- 
measures  of  Carbon  Ridge  show  a  thickness  of  3,500  feet,  which  does  not 
vary  essentially  from  the  development  found  on  Spring  Hill  and  is  within 
the  measurement  obtained  for  the  horizon  in  the  Diamond  Range,  where  the 
structural  relationships  with  both  the  upper  and  lower  beds  are  much  better 
determined.  The  Weber  conglomerate  has  been  regarded  as  dipping 
uniformly,  throughout  the  entire  development,  at  70°,  and  upon  this  assump- 
tion is  assigned  a  thickness  of  1,900  feet.  This  allows  the  conglomerate 
100  feet  less  than  the  estimated  thickness  in  the  Diamond  Range,  but 
here  the  uppermost  beds  are  known  to  be  buried  beneath  a  greater 
or  less  accumulation  of  tuffs  and  pumices.  That  there  is  about 
the  same  thickness  of  beds  and  great  similarity  in  the  nature  t.f 
the  sedimentation,  is  evident  from  a  comparative  study  of  the  two 
regions.  No  specially  favorable  locality  for  the  collection  of  fossils  was 
recognized  in  the  limestones,  mainly  because  none  were  sought,  but  through- 
out the  entire  series  of  beds  Coal-measure  forms  may  be  found.  Such 


174  GEOLOGY  OF  THE  EUliEKA  DJLSTKICT. 

species  as  Productus  semireticulatus,  P.  lonyispinus,  AtJiyris  subtilita,  and 
Spirifera  camerata  are  sufficient  to  establish  the  Carboniferous  age  of  the 
limestones,  and  their  position  beneath  the  Weber  conglomerate  assigns 
them,  beyond  question,  to  the  Lower  Coal-measures. 

On  PI.  ii  will  be  found  two  cross  sections  drawn  across  the  volcanic 
rocks  that  stretch  between  the  Hoosac  and  Pinto  faults,  separating  the  Car- 
boniferous strata  into  distinct  areas.  Both  sections  lie  between  the  two  gen- 
eral sections  E-F  and  I-K.  They  are  drawn  on  due  east  and  west  lines 
and  measure  a  little  over  2  miles  in  length.  Section  i,  atlas  sheet  vm,  passes 
just  south  of  the  Spring  Hill  limestone  body  and  crosses  the  hornblende 
andesite  nearest  its  broadest  expansion.  At  the  extreme  western  end  occurs 
a  small  exposure  of  Carboniferous  limestone,  only  a  few  hundred  yards  in 
length,  completely  surrounded  by  andesite.  As  shown  in  the  section  these 
andesites  extend  with  a  very  irregular  outline  for  a  long  distance,  beyond 
which  a  body  of  basalt  comes  in,  followed  by  limestone,  in  turn  followed 
by  pumices  overlain  and  buried  beneath  other  basalts.  These  latter  basalts 
give  out  on  the  steep  slopes  of  Dome  Mountain,  which  is  made  up  of  Ne- 
vada limestone,  lying  on  the  west  side  of  the  Pinto  fault. 

Section  n,  atlas  sheet  x,  is  drawn  so  as  to  show  the  great  body  of  Pinto 
Peak  rhyolite,  and  passes  just  south  of  the  summit  of  the  peak.  Along  this 
section,  between  the  two  great  meridional  lines  of  displacement,  none  other 
than  volcanic  rocks  reach  the  surface,  the  pumices  all  along  the  east  slope 
resting  against  the  upturned  Silurian  rocks  of  English  Mountain.  In  this 
section  the  Pogonip  limestone  is  seen  beyond  the  line  of  the  Hoosac  fault, 
but  its  direct  connection  with  the  fault  is  wholly  lost  by  outbursts  of  lava. 
By  reference  to  the  atlas  sheets  the  position  of  this  Pogonip  limestone  on 
the  west  side  of  the  fault  and  the  Carboniferous  limestone  on  the  east  side, 
will  be  readily  understood. 


U.S. GEOLOGICAL   SURVEY 


GEOLOG<   OF   EUREKA    DISTRICT    PLATE  I! 


Base-.  7000  fr. 


\.\\\  .\ga\\\-,\^<-^-./i-^-7v<Aiy  A  <'\/7A^y  A  \//Vv|/-y<yz./r<^A  A/v7.as 


^^S^^^ 


Bu.ie-.65noft. 


RubyHill 


Base^SOOft 


Prospect  P««k . 


Lovrr 
(intl  Mfa-ttirrs 


Secret  t'anat 
Shalr 


SCALE:   1600  ft  -  I   INCH 


GEOLOGICAL     SECTIONS. 
EUREKA    DISTRICT,  NEV. 


CHAPTEK  VI. 
GENERAL  DISCUSSION  OF  THE  PALEOZOIC  ROCKS. 

Paleozoic  shore-line.— Between  the  Wasatch  Range,  which  incloses  the 
Great  Basin  on  the  east,  and  the  western  border  of  the  Paleozoic  area  in 
central  Nevada,  the  sedimentary  beds  which  make  up  the  greater  part  of 
the  meridional  mountain  ranges  may,  as  regards  their  broader  divisions,  be 
fairly  well  correlated  with  each  other.  In  most  instances  paleoutological 
evidences  are  sufficient  to  determine  at  least  the  age  of  one  or  more  of  the 
great  bodies  of  limestone  usually  found  in  these  mountain  uplifts,  and  the 
sequence  of  strata  correlates  the  geological  position  of  overlying  and  under- 
lying beds.  Differences  in  the  character  of  these  sediments  exist,  but  they 
are  mainly  those  dependent  upon  distance  from  land  areas  and  depth  of 
water  in  which  the  material  was  originally  deposited.  Along  the  Wasatch 
the  sequence  of  strata  exhibits  much  the  same  physical  conditions  of  deposi- 
tion, and  the  horizons  may  be  recognized  and  their  positions  determined  in 
great  measure  by  similarity  of  sedimentation.  Over  a  large  part  of  central 
Utah  and  easteni  Nevada  the  beds  at  many  geological  horizons  indicate 
deep  water  or  off-shore  deposits  quite  unlike  those  of  corresponding  age 
found  both  to  the  east  and  west.  Here  and  there  over  this  region  some 
evidences  of  ancient  land  areas  may  be  found.  In  central  Nevada,  how- 
ever, there  occurs  throughout  the  beds  abundant  evidence  of  deposition 
in  shallow  seas.  The  western  limit  of  this  Paleozoic  ocean  across  the 
broadest  expansion  of  the  Great  Basin  was  not  far  from  longitude  117°  30'. 
In  width  it  measured  along  the  line  of  the  fortieth  parallel  nearly  300 
miles.  It  is  by  no  means  definitely  established  that  the  waters  rolled  un- 
broken, from  shore  to  shore,  across  this  broad  surface  free  from  all  land 

175 


176  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

barriers.  In  the  neighborhood  of  the  "East  Humboldt  Range  a  pre-Cam- 
brian  barrier  may  have  existed,  but  the  evidence  seems  rather  in  favor  of 
islands  rising  out  of  an  ocean,  which  stretched  across  the  entire  area  between 
broad  continental  regions. 

The  evidence  of  this  ancient  shore-line  rests,  as  has  been  pointed  out 
elsewhere,  upon  the  complete  and  unmistakable  differences  in  the  charac- 
ter of  the  beds  which  now  lie  uncomformably  upon  the  older  rocks  of 
the  continental  area  and  the  deposits  upon  the  ocean  bottom  to  the 
east.  Upon  this  pre-Cambrian  continent  no  Paleozoic  rocks  have  as  yet 
been  recognized  in  western  Nevada,  while  the  enormous  thickness  of 
uncomformable  strata  laid  down  since  the  uplifting  of  the  Paleozoic  area 
bears  ample  evidence  of  a  Jura-Trias  fauna,  as  shown  in  the  Piute  and 
West  Humboldt  ranges.  Again,  the  Mesozoic  rocks  of  the  Wasatch  and 
those  of  western  Nevada  bear  but  slight  resemblance  to  each  other,  either 
in  the  nature  of  the  material  deposited  or  in  the  character  of  the  life  repre- 
sented. Across  this  broad  intervening  area  all  evidences  of  Mesozoic  sedi- 
ments are  wanting  and  the  opposite  sides  are  sharply  contrasted  by  their 
physical  and  faunal  distinctions.  In  all  probability  the  Paleozoic  ocean  in 
Nevada  presented  an  indented  shore-line  with  a  general  northeast  and 
southwest  trend,  accompanied  by  outlying  islands  stretching  far  eastward 
and  rising  high  above  the  water  level.  This  ancient  coast  line  has  never 
been  traced,  consequently  its  outlines  are  most  indefinitely  determined.  It 
is  obscured  by  enormous  quantities  of  erupted  material,  in  places  literally 
mountain  high,  burying  for  long  distances  all  traces  of  preexisting  rocks. 

The  course  of  this  eruptive  action  was  in  great  part  determined  by  the 
profound  displacements  which  accompanied  the  elevation  and  transformed 
an  ocean  bottom  into  an  area  of  dry  land.  Measured  at  right  angles  to  the 
supposed  trend  of  the  shore  are  broad  areas  40  miles  in  width  across  which 
no  other  rocks  are  exposed  than  Tertiary  lavas  or  the  recent  Pleistocene 
deposits  of  the  valleys.  That  these  lavas  followed  the  trend  of  the  old 
shore  is  indicated  by  their  parallelism  with  the  mountain  uplifts,  and  that 
they  conformed  to  the  course  of  the  preexisting  continental  ranges  is  shown 
by  the  frequent  outcrops  of  Jura-Trias  rocks  projecting  above  the  vast  accu- 


PALEOZOIC  SHORE-LINE.  177 

mulations  of  volcanic  material.  Ranges  situated  eastward  of  the  supposed 
shore-line  expose  above  flows  of  rhyolite  long  ridges  of  quartzite  which 
have  been  referred  to  the  Paleozoic  age.  They  are  at  all  events  quite 
unlike  the  rocks  of  the  region  to  the  west.  In  a  study  of  the  geological  history 
of  continental  development  it  is  important  to  know  that  it  was  along  this 
ancient  shore-line  that  volcanic  activity  has  displayed  its  greatest  energy 
in  Nevada.  Upon  one  side  of  these  accumulated  lavas  is  found  an  enor- 
mous thickness  of  Paleozoic  strata  with  no  rocks  younger  than  the  Upper 
Coal-measures,  and  on  the  opposite  side  a  great  development  of  alpine 
Trias  and  Jura  is  seen  with  an  absence  of  the  Paleozoic  beneath  it.  These 
facts  furnish  strong  evidence  for  belief  in  the  existence  of  a  continental 
area  in  western  Nevada  during  Paleozoic  time.  To  the  south  the  shore- 
line probably  ran  out  toward  the  California  boundary;  to  the  northward  it 
may  be  traced  well  up  into  central  Nevada.  This  old  coast  line  is  a  region 
of  great  interest  and  one  which  would  well  repay  careful  investigation 
and  yield  valuable  geological  results. 

If  this  interpretation  of  observed  facts  is  correct,  the  degradation  of  the 
land  surface  during  Paleozoic  time  should  have  supplied  an  enormous  mass 
of  detrital  matter  to  the  ocean  to  the  east.  Now,  by  a  study  of  the  Eureka 
Paleozoic  strata,  this  is  precisely  what  is  found  to  be  the  condition  of  things. 
Along  this  coast  line  coarse  conglomerates  aud  mechanical  sediments  derived 
from  the  neighboring  land  areas  attest  the  fact  that  this  detrital  material 
must  have  come  not  only  from  the  west,  but  from  a  land  area  at  no  great  dis- 
tance. Along  the  shore  the  conglomerates  form  heavy  masses  of  material, 
indicating  littoral  deposits,  but  to  the  east  these  same  formations  gradually 
pass  into  fine  grained  sandstones,  the  beds  in  general  becoming  more 
uniform  in  character.  Exceptional  occurrences  of  coarse  and  rapidly  chang- 
ing material  can  be  found  in  eastern  Nevada,  but  for  the  most  part  they 
occupy  restricted  areas,  and  may  be  accounted  for  by  their  nearness  to  pre- 
Cambrian  islands.  All  evidence  goes  to  show  that  Eureka  was  situated  not 
far  from  this  western  boundary,  and  its  geological  record  affords  ample 
proof  of  elevation  and  depression  throughout  Paleozoic  time,  with  inter- 
vals of  shallow  water  and  nearness  of  land  areas  between  periods  of  rel- 
atively deeper  seas. 
MOW  xx 12 


178  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

Nature  of  Material.— The  30,000  feet  of  sediments  at  Eureka  between  the 
base  of  the  Prospect  Mountain  quartzite  and  the  summit  of  the  overlying 
Upper  Coal-measure  limestone  are  made  up  of  well  denned  bodies  of  sili- 
ceous, calcareous,  and  argillaceous  strata,  each  representing  a  distinctive 
epoch  in  the  geological  history  and  development  of  the  region.  These 
rock  masses  grouped  according  to  the  character  of  their  sediments  show 
clearly  the  relative  amount  of  the  three  classes  of  deposits  into  which  sub- 
aqueous sediments  may  be  divided. 

SILICEOUS. 

Feet. 

Prospect  Mountain  (Cambrian) 1, 500 

Eureka  quartzite  (Silurian) 500 

Diamond  Peak  quartzite  (Carboniferous) 3, 000 

Weber  conglomerate  (Carboniferous) 2, 000 

Total 7, 000 

CALCAREOUS. 

Prospect  Mountain  limestone  (Cambrian) 3, 050 

Hamburg  limestone  (Cambrian) 1, 200 

Pogonip  limestone  (Silurian) 2, 700 

Lone  Mountain  limestone  (Silurian) 1, 800 

Nevada  limestone  (Devonian) 6, 000 

Lower  Coal-measure  limestone  (Carboniferous) 3, 800 

Upper  Coal-measure  limestone  (Carboniferous) 500 

Total • 19, 050 

ARGILLACEOUS. 

Secret  Canyon  shale  (Cambrian) 1, 600 

Hamburg  shale  (Cambrian) 350 

White  Pine  shale  (Devonian) 2, 000 


Total 3, 950 

For  the  most  part  the  siliceous  formations  are  composed  of  pure  quartz- 
ites,  sandstones,  or  conglomerates,  interstratified  beds  of  foreign  material 
occupying  very  inferior  positions.  Oil  the  other  hand  the  calcareous  and 
argillaceous  deposits  are  more  or  less  interrupted  by  occasional  belts  of 
other  material  or  impure  layers  of  mixed  sediments  occurring  as  transi- 
tion beds. 


CHANGES  IN  OCEAN  LEVEL.  179 

In  the  Prospect  Mountain  limestones  occur  narrow  belts  and  lenticular 
bodies  of  clay  shales,  in  contradistinction  to  the  Pogonip,  which  is  charac- 
terized by  a  series  of  grits  and  sandstones.  Throughout  the  6,000  feet  of 
Nevada  limestone  the  pure  sandstone  beds,  taken  together,  would  scarcely 
measure  more  than  300  feet,  varying  from  25  to  100  feet  in  thick- 
ness, while  argillaceous  strata  are  exceptional  occurrences.  In  the  White 
Pine  shale,  which  is  mainly  argillaceous,  occur  several  beds  of  reddish 
sandstone  near  the  summit,  amounting  in  the  aggregate  to  several  hundred 
feet.  Deducting  from  the  limestone  epochs  the  beds  that  are  decidedly 
siliceous  and  argillaceous,  but  leaving  the  impure  strata,  which  are  mainly 
calcareous,  we  find  the  aggregate  thickness  of  the  three  classes  of  sedi- 
ments as  follows :  Argillaceous,  3,000;  siliceous,  9,000;  calcareous,  18,000; 
or  as  1 :  3  :  6. 

The  Prospect  Mountain  quartzite  throughout,  at  least  so  far  as  it  is 
exposed  at  Eureka,  can  hardly  be  otherwise  than  an  off-shore  deposit.  The 
base  of  the  formation  is  largely  composed  of  coarse  conglomerates,  made 
up  of  a  great  variety  of  unassorted  siliceous  pebbles,  while  the  finer  beds 
nowhere  present  any  considerable  thickness,  and  show  evidence  of  strong 
currents.  It  is  certain  that  such  material  could  not  have  been  transported 
for  any  great  distance  in  deep  water.  At  the  top  transition  beds  of 
siliceous  sands  pass  rapidly  into  the  limestones  of  the  Prospect  Mountain 
series,  which,  across  a  thickness  of  3,000  feet,  carries  but  one  heavy,  per- 
sistent belt  of  clay  shale. 

The  Secret  Canyon  shale,  a  remarkably  uniform  horizon  throughout 
its  great  thickness,  presents  a  wholly  different  character  of  deposits  from 
the  two  underlying  formations.  Nowhere  are  there  any  evidences  of  rapid 
deposition.  Following  the  latter  formation  comes  a  second  belt  of  lime- 
stone, with  occasional  beds  of  siliceous  material  in  place  of  the  clayey  beds 
found  in  the  Prospect  Mountain  limestone.  Above  this  second  belt  of  lime- 
stone occurs  the  Hamburg  shale,  indicating  a  complete  change  in  the  sedi- 
mentation and  reproducing  conditions  like  those  of  the  Secret  Canyon 
period,  although  only  350  feet  in  thickness.  In  the  character  of  its  deposits 
it  changes  more  rapidly  and  shows  unstable  conditions  in  its  mode  of  sedi- 
mentation. Next  in  order  is  found  the  widespread  Pogonip  limestone,  the 


180  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

• 

base  of  the  Silurian  system.  The  transition  beds  show  a  constant  change 
in  the  deposit,  all  of  them  presenting  more  or  less  mixed  material,  develop- 
ing into  argillites,  grits,  and  arenaceous  schists,  finally  passing  into  distinctly 
bedded  uniform  limestone.  Overlying  the  Pogonip  rests  the  Eureka 
quartzite,  about  500  feet  in  thickness,  but  singularly  uniform  in  its  material 
and  wholly  unlike  all  the  other  siliceous  formations,  being  formed  of  pure 
white  siliceous  grains  completely  metamorphosed  to  quartzite.  Through 
this  formation,  except  at  its  base,  there  is  a  marked  absence  of  mixed  sedi- 
ments and  off-shore  material.  All  the  conditions  of  deposition  suggest  deeper 
water  and  a  quieter  sea  bottom.  Immediately  above  the  Eureka  quartzite 
comes  an  immense  development  of  limestone,  with  occasional  interbedded 
sandstones  at  varying  intervals,  but  comparatively  free  of  earthy  matter. 
Taken  together,  the  Lone  Mountain  and  Nevada  formations  which  make  up 
these  limestones  measure  nearly  8,000  feet  in  thickness,  apparently  laid 
down  without  any  decided  break  in  the  conditions  of  deposition,  although 
the  accumulation  of  such  a  vast  amount  of  calcareous  sediment  must  have 
occupied  a  long  period  of  time.  That  it  was  sufficient,  notwithstanding  its 
uniformity,  to  bring  about  marked  changes  in  the  life  of  the  sea  is  shown 
by  its  faunal  development.  There  exists  no  greater  break  in  the  character 
of  the  sediments  than  that  found  between  the  Nevada  limestone  and  the 
overlying  White  Pine  shale.  The  change  from  the  calcareous  deposit  of  a 
quiet  ocean  with  a  marine  fauna  to  an  argillaceous  deposit  full  of  carbon- 
aceous material  with  evidence  of  cross-bedding,  rapid  currents,  and  shallow 
water  recalls  a  retreating  sea  and  the  proximity  of  a  land  surface. 

Remains  of  vegetable  life  are  of  rare  occurrence  in  the  Paleozoic 
rocks  of  the  Great  Basin  and  eastward  of  the  East  Humboldt  Range  are 
quite  unknown.  The  White  Pine  shale,  both  at  Eureka  and  White  Pine, 
carries  innumerable  fragmentary  bits  of  twigs  and  stems  throughout  the 
entire  formation,  although  for.  the  most  part  too  poorly  preserved  for 
specific  determination,  yet  indicating  land  areas  throughout  a  long  period 
of  time.  Over  the  White  Pine  shale  was  laid  down  the  Diamond  Peak 
quartzite,  a  uniform  deposit  of  fine  grained  siliceous  material  without  any 
special  evidence  of  the  proximity  of  land,  either  in  the  life  or  mode  of 
deposition.  An  interstratified  bed  of  limestone  carrying  Productus  semire- 


LAND  AREA  IN  THE  CARBONIFEROUS. 

ticulatus  establishes  the  Carboniferous  age  of  the  quartzite.  After  the  Dia- 
mond Peak  quartzite  attains  a  thickness  of  3,000  feet,  a  change  sets  in  with 
alternating  beds  of  coarse  shales  and  conglomerates  more  or  less  mixed 
with  calcareous  sediments,  the  entire  series  being  admirably  exposed  at  the 
base  of  the  Lower  Coal-measures  on  the  southwest  slope  of  Richmond 
Mountain.  Here  we  find  positive  evidence  of  the  existence  of  fresh-water 
shells  associated  in  the  same  beds  with  plant  life  fairly  well  preserved, 
although  specific  determinations  are  impossible.  This  is  the  only  instance 
yet  discovered  of  the  existence  of  fresh-water  species  in  the  Paleozoic 
rocks  of  the  Great  Basin,  and  points  conclusively  to  the  existence  of  a  land 
surface  at  no  great  distance  and  long  after  the  White  Pine  shales  had  been 
buried  beneath  3,000  feet  of  sands.  Even  if  they  had  been  deposited  in 
an  estuary  and  washed  into  their  present  position  by  rapid  currents  the 
land  area  could  not  be  far  away.  This  group  of  rocks  carrying  a  fresh- 
water fauna  soon  becomes  submerged  beneath  the  limestones  of  the  Lower 
Coal-measures,  which  occupy  such  widespread  areas  of  the  Great  Basin  and 
which,  so  far  as  the  physical  conditions  of  deposition  are  concerned,  closely 
resemble  the  Nevada  limestone. 

Additional  evidence  of  land  areas  during  the  Carboniferous  is  found  at 
Pancake  and  Bald  Mountain,  a  somewhat  similar  series  of  strata  occurring 
in  both  localities,  with  well  developed  coal  seams,  bituminous  shales,  and 
evidences  of  plant  life  both  above  and  below  the  coal. 

Next  in  turn  overlying  the  Lower  Coal-measures  occurs  the  Weber 
conglomerate,  a  formation  2,000  feet  in  thickness,  of  coarse  siliceous  mate- 
rial made  up  of  pebbles  varying  in  size,  composed  of  quartz,  jasper,  chert, 
and  hornstone,  unquestionably  an  off-shore  deposit  in  shallow  water.  Such 
coarse  material  could  not  have  been  transported  any  great  distance. 
The  conglomerates  of  Agate  Pass,  in  the  Cortez  Range,  and  at  Moleen 
Peak  present  identical  physical  conditions  with  evidences  of  the  same 
off-shore  deposits.  To  the  eastward,  removed  from  the  continental  area, 
the  sediments  of  this  epoch  become  finer  grained  and  arenaceous  in  tex- 
ture. How  long  a  time  was  occupied  in  the  accumulation  of  this  great 
thickness  of  siliceous  pebbles  it  is  of  course  impossible  to  say.  Finally  it 
was  followed  by  a  submergence  accompanied  by  a  deposition  of  limestone 


182  GEOLOGY  OF  THE  EUREKA  DISTBICT. 

characterized  by  a  fauna  which  did  not  differ  in  any  marked  degree  from 
the  underlying  limestone  of  the  Lower  Coal-measures. 

The  Upper  Coal-measures  indicate  once  more  that  a  deeper  sea  swept 
over  the  siliceous  beds  as  the  limestones  of  this  upper  horizon  were  laid 
down  over  a  wide  area,  presenting  great  uniformity  in  composition  and 
marine  life.  Of  these  overlying  rocks  we  have  only  500  feet  at  Eureka, 
and  with  them  the  geological  record  of  Paleozoic  time  comes  to  an  end. 

From  this  recapitulation  of  the  record  of  the  Eureka  rocks,  it  is  evident 
that  they  were  laid  down  under  very  varying  physical  conditions.  In  its 
broader  outlines  this  sequence  of  strata  may  be  correlated  with  the  rocks 
of  the  Great  Basin  stretching  as  far  eastward  as  the  Wasatch,  with  this 
difference,  that  at  Eureka  there  may  be  seen  immense  thicknesses  of  shal- 
low water  sediments  derived  from  a  continental  area  to  the  west,  whereas 
in  going  eastward  evidences  of  deeper  waters  are  met  with,  the  material 
being  more  uniform  in  character  and  deposited  in  a  quieter  sea.  In  the 
latter  rocks  there  is,  so  far  as  yet  recognized,  a  marked  absence  of  argil- 
laceous and  calcareous  deposits  enriched  with  plant  remains  and  fresh- 
water shells. 

Naturally,  it  is  the  siliceous  formations  that  exhibit  the  greatest  litho- 
logical  contrasts,  and  in  going  eastward  from  off-shore  to  deep-water  de- 
posits it  seems  highly  probable  that  Cambrian  quartzite  in  places  approxi- 
mates the  Eureka  quartzite  in  appearance,  and  may  possibly  have  been 
mistaken  for  it.  In  the  same  manner  the  widely  deposited  Weber  formation 
passes  from  the  coarse  conglomerate  as  represented  at  Eureka  into  fairly 
uniform  beds  of  quartzite  or  sandstone.  Owing  to  its  geological  position 
between  the  Upper  and  Lower  Coal-measures,  it  becomes  a  comparatively 
easy  matter  to  correlate  the  Weber  quartzite  of  Eureka  and  Agate  Pass 
with  sandstones  farther  eastward  or  the  Weber  shales  of  still  other  localities. 
Again,  the  Weber  epoch,  when  composed  of  fine  siliceous  grains  and  car- 
rying a  considerable  amount  of  calcareous  material,  may  be  difficult  to  sep- 
arate from  the  overlying  and  underlying  limestones. 

Paieontoiogicai  Divisions.— As  already  mentioned,  the  30,000  feet  of  strata 
have  been  divided  into  the  four  great  periods  of  Paleozoic  time,  and  at 
Eureka  they  have  again  been  subdivided  into  epochs  mainly  based  upon 


PALEONTOLOGICAL  DIVISIONS.  183 

the  lithological  character  of  their  sediments.  For  the  advancement  of  geo- 
logical science  it  is  necessary  for  the  geologist  and  paleontologist  to  agree 
upon  some  broad  principle  governing  the  division  of  Paleozoic  time  and  for 
the  purpose  of  correlating  the  strata  of  one  locality  with  those  of  another. 
From  long  experience  it  is  found  that  a  division  based  upon  paleontological 
data  is  the  only  one  which  will  meet  the  requirements  over  widely  separated 
areas  of  the  globe.  Structural  breaks,  based  upon  unconformities  of  deposi- 
tion or  lithological  distinctions,  determined  by  manner  of  occurrence,  may 
meet  the  needs  of  local  geological  provinces  far  better  than  a  paleontologi- 
cal classification,  but  for  broader  continental  areas  they  are  far  too  restricted 
for  the  purposes  of  correlation.  The  broad  divisions  at  Eureka  are  based 
upon  paleontological  evidence.  In  the  6,000  feet  of  Cambrian  sediments 
above  the  base  of  the  Olenellus  shale,  the  Lower,  Middle,  and  Upper  Cam- 
brian horizons  are  all  well  represented  by  characteristic  faunas.  The  line 
between  the  Cambrian  and  Silurian  is  drawn  just  above  the  Hamburg  shale, 
and  is  determined  wholly  by  faunal  development.  In  the  interstratified 
grits  and  limestones,  which  bring  in  the  Pogonip,  animal  life  undergoes 
a  gradual  change  with  the  extinction  of  an  old  fauna  and  the  coming  in  of 
a  new  one.  Without  any  marked  physical  disturbance  and  a  continuance 
of  limestone  strata,  a  commingling  of  forms  is  to  be  expected.  A  few  of  the 
more  persistent  Potsdam  types  are  found  at  the  base  of  the  Pogonip,  but  a 
characteristic  Chazy  fauna  rapidly  takes  the  place  of  the  life  found  below 
the  Hamburg  shale.  At  the  top  of  the  Pogonip  the  Trenton  epoch  is  fore- 
shadowed by  the  presence  of  a  number  of  species  characteristic  of  that  hor- 
izon on  the  Atlantic  border.  The  Eureka  quartzite  affords  no  evidence  of 
animal  life,  but  immediately  upon  the  renewal  of  conditions  favorable  to 
limestone  deposition  several  of  the  same  Trenton  species  reappear,  strongly 
reinforced  by  a  group  of  forms  decidedly  Trenton  in  its  aspect,  while  by 
far  the  greater  part  of  the  life  observed  below  the  quartzite  has  passed  away 
forever.  The  Trenton  is  followed  by  a  monotonous  limestone  2,000  feet  in 
thickness,  carrying  a  few  scattered  corals,  Hali/sitcx  catcnulatus  being  suffi- 
ciently characteristic  to  identify  the  beds  as  belonging  to  the  Niagara.  The 
Silurian  of  Eureka  consists,  then,  of  two  heavy  bodies  of  limestone  with  dis- 
tinct faunas,  separated  by  a  dense  white  quartzite. 


184  GEOLOGY  OF  THE  EUEEKA  DISTRICT. 

Without  any  discordance  in  deposition  or  chemical  change  in  composi- 
tion of  the  limestone,  the  next  great  period  of  Paleozoic  time  as  deter- 
mined by  its  life  comes  in,  marked  by  the  appearance  of  Atrypa  reticwlaris 
and  associated  species,  followed  by  an  abundant  and  strikingly  character- 
istic Devonian  fauna,  which  is  maintained  to  the  top  of  the  Nevada  lime- 
stone. A  decided  change  in  the  nature  of  the  sediments  brings  in  the 
White  Pine  shale  with  its  peculiar  fauna  and  flora,  but  still  characterized  by 
its  Devonian  aspect. 

The  Diamond  Peak  quartzite  carries  the  first  evidence  of  an  unques- 
tioned Carboniferous  life,  as  shown  by  one  or  two  Coal-measure  species  in 
an  interbedded  limestone.  From  the  summit  of  this  horizon  upward  the 
easily  recognized  Carboniferous  fauna,  as  seen  in  so  many  ranges  over  the 
Great  Basin,  continues  to  the  summit  of  the  Paleozoic  sediments,  through 
two  heavy  masses  of  limestone  and  an  intermediate  quartzite  or  sandstone. 

Physical  Divisions.— There  can  be  no  question  that  a  geologist  making 
a  division  of  the  Paleozoic  rocks  of  Eureka,  if  he  were  guided  solely  by 
the  physical  conditions  found  there,  would  carry  the  first  period  up  to  the 
Eureka  quartzite.  In  doing  this  he  would  be  drawing  a  line  in  accordance 
with  the  most  important  break  to  be  found  in  the  faunal  development  of  the 
lower  Paleozoic  rocks,  the  greatest  change  occurring  between  the  life  found 
just  below  the  Eureka  quartzite  and  that  coming  in  a  short  distance  above  it. 
It  would  be  a  line  in  agreement  with  a  sharp  lithological  change  which 
brought  about  conditions  detrimental  to  life,  and  at  the  same  time  would 
make  a  separation  which  would  coincide  with  the  one  unconformity  by 
deposition  as  yet  recognized  in  the  record  of  the  Lower  Paleozoic  rocks  in 
the  Great  Basin.  The  conformable  series  of  limestones  and  shales  between 
the  Prospect  Mountain  quartzite  and  the  Eureka  fall  naturally  into  one 
grand  period.  As  already  pointed  out,  the  Hamburg  shale  separates  the 
Cambrian  from  the  Silurian  at  Eureka,  but  in  other  localities  this  shale  is 
entirely  wanting,  the  Hamburg  limestone  passing  up  into  thePogonip  with- 
out any  lithological  break.  Even  at  White  Pine,  only  40  miles  away,  the 
shale  is  wanting.  In  working  out  the  structural  relations  of  the  entire  series 
of  Paleozoic  sediments  over  much  of  the  Great  Basin,  one  of  the  chief  diffi- 


PHYSICAL  DIVISIONS  AT  EUEEKA.  185 

culties  has  been  to  connect  the  Cambrian  and  Lower  Silurian  rocks  below 
the  Eureka  quartzite  with  the  Upper  Silurian  and  Devonian  above  it. 

Again,  from  a  physical  point  of  view  there  are  obvious  reasons  for 
linking  together  in  one  group  the  enormous  development  of  limestones 
lying  between  the  Eureka  quartzite  and  the  Diamond  Peak  quartzite.  So 
imperceptible  is  the  transition  in  physical  characters  between  the  Lone 
Mountain  beds  of  the  Silurian  and  the  Nevada  limestone  of  the  Devonian 
that  no  line  can  be  sharply  drawn,  and  while  the  fauna  slowly  undergoes 
change  there  are  hundreds  of  feet  of  sediments  that  might  as  well  be  placed 
in  one  as  the  other  of  the  two  formations.  Atrypa  reticularis  and  other  species 
found  near  the  base  of  the  Devonian  have  at  other  localities  been  obtained, 
associated  with  species  regarded  as  belonging  to  the  Silurian. 

With  the  coining  in  of  the  Diamond  Peak  epoch  another  great  change 
takes  place  in  the  physical  conditions,  and  with  it  a  marked  faunal  break, 
which  brings  in  the  Carboniferous  period.  From  the  Diamond  Peak  quartz- 
ite to  the  summit  of  the  Paleozoic  rocks  the  beds  form  one  natural  group, 
no  matter  from  what  point  of  view  they  may  be  considered.  The  litholog- 
ical  distinctions  hold  good  over  wide  areas. 

From  the  standpoint  of  physical  geology  the  record  shows  three  grand 
divisions:  first,  one  with  the  Prospect  Mountain  quartzite  at  the  base,  fol- 
owed  by  a  series  of  limestones  and  shales;  second,  beginning  with  the 
Eureka  quartzite,  followed  by  another  series  of  limestones  to  the  top  of 
the  Devonian;  third,  a  great  quartzite  belt,  followed  in  turn  by  a  lime- 
stone, a  conglomerate,  and  a  second  limestone. 

LOWER   PALEOZOIC    IN   ADJOINING   REGIONS. 

After  the  completion  of  the  field  work,  upon  revisiting  several  of  those 
ranges  in  the  Great  Basin  where  the  descriptions  given  of  the  lower  Paleo- 
zoic sections  differed  essentially  from  the  sections  exposed  at  Eureka,  it 
was  found  that,  as  regards  sequence  of  beds,  they  stood  singularly  in  accord 
and  could  easily  be  correlated  with  those  of  the  latter  locality.  A  knowl- 
edge of  the  sedimentary  beds  at  Eureka  serves  to  unravel  in  neighboring 
ranges  several  knotty  problems  previously  not  clearly  understood,  and  to 
show  that  similar  physical  conditions  existed  over  a  wide  area  of  ocean 


186  GEOLOGY  OF  THE  EUKEKA  DISTRICT. 

bottom.  For  comparative  purposes,  therefore,  it  may  be  well  to  introduce 
here,  with  more  or  less  detail,  some  descriptions  of  the  geological  structure 
and  occurrences  elsewhere  in  the  Great  Basin  of  those  portions  of  the  lower 
Paleozoic  horizons  that  happen  to  be  well  represented  at  Eureka.  The  sec- 
tion at  Eureka  may  be  taken  as  a  standard. 

The  Oquirrh  Mountains.— In  the_  Oquirrh  Mountains,  the  first  range  west  of 
the  Wasatch,  the  Olenellus  horizon  has  been  identified  in  a  thin  bed  of 
yellow  shale  conformably  overlying  a  reddish  white  quartzite  of  unknown 
thickness,  above  which  occur  from  3,000  to  4,000  feet  of  limestone  carry- 
ing Lower  Carboniferous  and  Coal-measure  fossils.  The  geological  posi- 
tion of  the  shale  bed  is  determined  by  the  following  species :  Lingulella  ella, 
Olenellus  gilberti,  and  Bathyuriscus  prodmta.  Mr.  S.  F.  Emmons,1  while 
engaged  upon  the  Geological  Exploration  of  the  Fortieth  Parallel,  exam- 
ined this  range  and  shrewdly  suggested,  from  orographic  evidences,  that  a 
fault  existed  between  the  shales  and  overlying  limestones — a  structure 
which  would  be  in  accord  with  the  observed  facts  brought  out  at  Eureka. 

The  Highland  Range.— The  Highland  Range,  about  125  miles  south  of 
Eureka,  presents  a  geological  structure  in  many  respects  similar  to  Pros- 
pect Mountain,  although  by  no  means  as  simple  or  furnishing  an  unbroken 
section  of  equal  extent.  At  Pioche,  at  one  time  a  nourishing  mining  town, 
situated  on  an  eastern  spur  of  the  main  range,  Mr.  E.  E.  Howell  obtained 
two  species  of  the  genus  Olenellus,  which  Mr.  F.  B.  Meek'  described  as 
0.  gilberti  and  0.  howelli.  According  to  Mr.  Howell3  they  occur  in  a  red- 
dish yellow  arenaceous  shale  about  400  feet  in  thickness,  overlying  1,200 
feet  of  quartzite.  In  the  published  section  the  shale  is  represented  as  con- 
formably overlain  by  gray  limestone,  which  he  regarded  as  of  Carbonifer- 
ous age,  although  no  paleontological  evidence  was  presented.  If  these 
strata  are  conformable  it  would  seem  highly  probable,  from  the  known 
sequence  of  beds  at  Eureka  as  well  as  elsewhere  in  the  Highland  Range, 
that  the  overlying  limestone  belongs  to  the  Prospect  Mountain  epoch. 

'U.  8.  Geol.  Explor'n  40th  Par.,  vol.  2,  Descriptive  Geology,  p.  444. 

3  Geographical  Surveys  West  of  One  hundredth  Meridian,  vol.  3,  p.  182. 

3  Op.  oit.  vol.  3,  p.  258. 


HIGHLAND  RANGE.  187 

Subsequently,  during  the  summer  of  1885,  Mr.  C.  D.  Walcott  studied 
the  structure  at  Pioche,  and  also  made  an  extended  examination  of  the  Cam- 
brian and  Silurian  rocks  of  the  main  uplift  of  the  Highland  Range.  He 
estimated  about  7,000  feet  of  strata  between  the  summit  of  the  Prospect 
Mountain  quartzite  and  the  base  of  the  Eureka  quartzite,  as  against  9,000 
feet  at  Eureka.  Between  Bennett  Spring  and  Stampede  Gap  the  lower  mem- 
bers of  the  group  were  carefully  measured,  the  section  presenting  the  beds 
more  in  detail  and  showing  greater  variation  of  sedimentation,  with  more 
interbedded  siliceous  material  than  observed  in  the  corresponding  horizons 
at  Eureka.  Detailed  lithological  sections,  however,  across  the  Cambrian 
rocks  are  perhaps  of  little  value,  owing  to  the  rapid  changes  in  the  charac- 
ter of  the  beds.  Sections  made  across  Prospect  Mountain  show  considera- 
ble difference  in  detail,  but  agree  substantially  in  general  features.  The 
section  at  Bennett  Spring  is  as  follows  :' 

•  Feet. 

1.  Dark  reddish  brown  quartzite,  evenly  bedded  and  ripple-marked  in  places .          350 

2.  Bluish  gray  limestone 35 

Fossils :  Olenellus  gilberti. 

3.  Buff  argillaceous  and  arenaceous  shales,  more  or  less  solid  near  the  base 

and  laminated  in  the  upper  portions 80 

Fossils:  Annelid  trails  and  fragments  of  Olenellus  in  the  lower  part; 
higher  up  the  heads  of  Olenellus  gilberti  and  0.  iddingsi  occur  in 
abundance. 

4.  Light  colored  gray  limestone  and  bluish  black  limestone 16 

5.  Sandy,  buff  colored  shale 40 

Fossils :  Annelid  trails,  Cruziana,  sp.  ? 

6.  Dark  bluish  black  limestone 46 

7.  Finely  laminated  buff  argillaceous  shale 80 

Fossils :  Hyolithes  billingsi  and  Ptychoparia  piochensis. 

8.  Gray  to  bluish  black  compact  limestone 18 

9.  Buff  arenaceous  shales 64 

10.  Compact  cherty  limestone 

11.  Compact  shaly  sandstone  in  massive  layers 40 

12.  Hard  siliceous  gray  limestone,  almost  quartzite  at  base 12 

13.  Yellow  to  buff  sandy  shales . , 70 

14.  Bluish  black  limestone 

'Second  Contribution  to  the  Studies  on  the  Cambrian  Faunas  of  North  America.    U.S.  G«oL  Surv. 

Bull.  No.  30,  1886,  p.  34. 


188  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

Feet. 

15.  Yellow  to  buff  sandy  shales 40 

16.  Bluish  black,  hard,  compact  limestone  12 

Fragments  of  fossils. 

17.  Shaly  sandstone  in  massive  layers 52 

18.  Gray  arenaceous  limestone 2 

19.  (a)  Buff  sandy  shale 40 

(b)  Gray  arenaceous  limestone 30 

(c)  Sandy  calcareous  shale 3 

73 

20.  (a)  Massive  bedded  bluish  gray  limestone 200 

Fragments  of  fossils. 

(b)  Compact  gray  siliceous  limestone,  almost  quartzite  in  places 400 

(c)  Bluish  black  evenly  bedded  limestone 6 

606 
Strike  K  30°  W.,  dip  10°  E. 

21.  Buff  to  pinkish  argillaceous  shale,  with  fossils,  and  a  few  interbedded 

layers  of  limestone  from  3  to  15  inches  thick 125 

Fossils:  Eocystitesf?  longidactyltis,  Lingulella  ella,  Kutorginapannula, 
Hyolithes  billingsi,  Ptychoparia  piochensis,  Oknoides  typicalis, 
Bathyuriscws  howelli,  aiid  B.  producta. 

22.  Massive  bedded  siliceous  limestone,  weathering  rough  and  broken  into 

great  belts  200  to  300  feet  thick  by  bands  of  color  in  light  gray,  dark 
lead,  to  bluish  black ;  on  some  of  the  cliff  faces  the  weathered  surface 
is  reddish 1, 570 

23.  Bluish  black  limestone  in  massive  strata  that  break  up  into  shaly  layers 

on  exposure  to  the  weather.  The  latter  feature  is  less  distinct  850  feet 
up,  and  the  limestone  becomes  more  siliceous,  with  occasional  shaly 

beds 1, 430 

Fossils :  Near  the  summit  specimens  were  found  that  are  referred  to 
Ptychoparia  minor. 

The  upper  limestone  on  the  line  of  the  section  is  not  favorable  for  the 
preservation  of  organic  remains,  but  the  same  horizon  a  short  distance  to 
the  southward  yielded  a  fauna  similar  to  that  from  the  Upper  Cambrian  at 
Eureka,  two  species  being  identically  the  same,  while  two  others,  Beller- 
ophon  antiquatus  and  Dicellocephalus  pepinensis,  occur  in  the  Potsdam  sand- 
stone of  Wisconsin. 

Timpahute  Range.— Mr.  G.  K.  Gilbert  reports  from  the  southern  end  of 
Timpahute  Range  a  section  over  2,300  feet  in  thickness,  which,  taken 


8ILVEE  PEAK.  189 

tog-ether  with  paleontological  evidence,  may  be  readily  correlated  with  the 
lower  part  of  the  Cambrian  of  Eureka  and  the  Highland  Range.  The 
thicknesses  are  estimated.  The  section  is  as  follows  r1 

South  end  of  Timpahute  Range.    Eastern  Nevada. 

Feet. 

1.  Heavy  bedded  gray  limestone,  light  and  dark 400 

2.  Yellow  argillaceous  shale: 

(a)  Yellow  shale 350 

(6)  Yellow  sandstone 75 

(c)  Yellow  and  green  shale,  with  fillets  of  fossiliferous  limestone 

(Conocoryphe) 500 

925 

3.  Purple  ripple-marked  vitreous  sandstone,  with  bands  of  siliceous  shale. . .      1, 000 

Total 2,326 

The  lower  bed  corresponds  to  the  Prospect  Mountain  quartzite.  In  the 
overlying  yellow  shale  he  collected  a  few  fossils,  determined  by  Mr.  Wal- 
cott  as  Olenellus  gilberti  and  0.  iddingsi. 

silver  Peak.— Still  farther  west,  in  a  bed  of  yellowish  brown  limestone 
with  intercalated  gray  argillaceous  shales  at  Silver  Peak,  a  small  collection 
of  fossils  was  made,  which  Prof.  J.  D.  Whitney2  placed  before  the  Cali- 
fornia Academy  of  Sciences  as  early  as  1866.  At  that  time  he  regarded 
them  as  Upper  Silurian  or  Devonian.  Quite  recently  Mr.  Walcott3  has  ex- 
amined the  collection  and  determined  the  following  species: 

Archseocyathus  atlanticus.  Kutorgina  (like  K.  cingulata). 

Archseocyathus,  undt.  sp.  Hyolithes  princeps. 

Ethmophyllum  whitneyi.  Olenellus  gilberti. 
Sti ephochetus ?  sp.? 

A  number  of  species  proved  to  be  identical  with  those  found  on  the 
coast  of  Labrador  and  the  horizon  is  evidently  the  equivalent  of  the  Georgia 
or  Lower  Cambrian  formation  of  Prospect  Mountain.  He  also  determined 
Olenellus  gilberti  as  closely  resembling  Olenellus  tltowpsoni  from  L'Anse  au 
Loup. 

'Geographical  Surveys  West  of  One  hundredth  Meridian,  Washington,  1875,  vol.  3,  p.  169. 
J  Proc.  Cal.  Acad.  Sci.,  vol.  3,  p.  270. 

3  Second  Contribution  to  the  studies  on  the  Cambrian  Faunas  of  North  America.  U.  S.  GeoL 
Snrv.  Bull.  No.  30,  1886,  p.  38. 


190  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

Silurian  and  Devonian.— Exposures  of  Silurian  and  Devonian  rocks  present- 
ing a  development  of  strata  at  all  comparable  with  the  Eureka  section  are 
known  in  but  few  localities  in  the  Great  Basin,  and  nowhere  are  the  struc- 
tural relations  of  the  Cambrian,  Silurian,  and  Devonian  so  clearly  brought 
out  as  here.  Nevertheless,  numerous  mountain  uplifts  present  so  many 
partial  exposures  of  the  Eureka  section  that  the  evidence  is  sufficient  to  de- 
termine the  same  succession  of  strata  over  a  wide  area  of  the  Great  Basin. 

Geological  Section,  white  Pine.— In  the  year  1872  the  writer1  visited  White 
Pine  and  Eureka  and  established  the  identity  of  a  great  thickness  of  the 
Pogonip  beds  at  both  places.  The  fossils  collected  at  that  time  from  Pogo- 
uip  Mountain,  White  Pine,  and  the  hills  east  of  the  Jackson  Mine  at  Eureka, 
were  submitted  to  Messrs.  Hall  and  Whitfield  and  shown  by  them  to  be 
specifically  identical.  At  that  time,  however,  neither  tliu.  base  nor  the 
summit  of  the  epoch  was  clearly  defined  and  not  until  after  the  thorough 
survey  of  the  Eureka  district  were  their  exact  limitations  known,  nor  could 
they  be  determined  until  after  the  collection  of  a  large  amount  of  paleonto- 
logical  material.  Topographically,  Pogonip  Ridge  holds  much  the  same 
relation  to  the  White  Pine  Mountains  that  Prospect  Ridge  does  to  the  Eu- 
reka Mountains.  It  forms  the  most  prominent  uplift  in  that  district,  occur- 
ring as  a  sharp  longitudinal  ridge,  the  highest  point  attaining  an  elevation 
of  nearly  10,800  feet  above  sea  level.  Across  this  ridge  the  oldest  sedi- 
mentary beds  of  the  district  lie  inclined  at  high  angles  to  the  east,  the 
northern  end  being  made  up  almost  wholly  of  Silurian  rocks.  The  structure 
is  simple,  the  beds  which  trend  obliquely  across  the  ridge  being  easily  fol- 
lowed from  the  summit  of  the  peak  to  the  northern  base. 

After  the  completion  of  the  work  at  Eureka  Mr.  C.  D.  Walcott  made  a 
careful  examination  of  the  Silurian  rocks  at  White  Pine  where,  according  to 
him,  the  Pogonip  strata  measure  over  5,000  feet  in  thickness.  The  oldest 
strata  identified  by  their  organic  remains  are  the  beds  at  the  base  of 'the 
Hamburg  limestone,  several  species  being  identical  with  those  from  the 
corresponding  horizon  at  Eureka.  These  Hamburg  limestone  beds,  several 
hundred  feet  in  thickness,  are  cut  off  by  a  fault  bringing  them  in  contact 

1 U.  S.  Geological  Exploration  of  the  Fortieth  Parallel,  vol.  2,  Descriptive  Geology,  pp. 
542  and  547. 


SILURIAN  AT  WHITE  PDTE.  191 

with  a  heavy  bed  of  quartzite  that  forms  the  western  central  spur  of  Po- 
gonip  Ridge.  As  the  Hamburg  shales  are  wanting,  the  Hamburg  limestone 
and  the  included  fauna  continue  to  the  base  of  the  Pogouip.  Here,  as  well 
as  at  Eureka,  the  base  of  the  Pogonip  is  determined  by  a  commingling  of 
Cambrian  and  Silurian  species,  the  line  of  demarcation  resting  wholly  upon 
paleontological  evidence.  Mr.  Walcott  examined  the  beds  from  the  lower 
quartzite  across  the  Pogonip,  Eureka  quartzite,  and  Lone  Mountain,  until 
the  upper  beds  of  the  latter  epoch  were  lost  beneath  the  detritus  of  the 
plain. 

The  section  is  as  follows: 

NIAGARA. 

Feet. 

1.  Yellowish  shaly  limestone 50 

2.  Light  colored,  massive  bedded   siliceous  limestone,  with  plates  of  cri- 

noids,  etc 650 

3.  Light  blue  siliceous  limestones  with  impressions  of  corals,  Halysites  caten- 

ulatus,  Stromatopora  f 150 

4.  Light  gray  siliceous  limestones 50 

TRENTON. 

5.  Evenly  bedded  pure  bluish  gray  limestones 50 

Fossils:  Cystidian  plates,  Bryozoa  3  sp.,  Rhynchonella  capax,  Trinu- 
cleus  concentricus,  Streptorkynchus  filitexta,  Orthis  subquadrata, 
Pterinea. 

6.  Dark  colored  siliceous  limestone  in  massive  beds 500 

EUREKA. 

7.  Light  vitreous  quartxite,  ferruginous  near  the  base 350 

POGONIF. 

8.  Dark  blue  and  black  limestones,  with  numerous  shaly  belts,  characterized 

by  the  fossils  of  the  Upper  Pogonip  as  seen  at  Eureka,  nearly  all  the 
genera  being  recognized,  with  the  exception  of  Receptaculites 900 

9.  Dark  evenly  bedded  limestones,  with  more  or  less  siliceous  bauds 4, 300 

Fossils:  Acrotreta  gemma,  Ill&nus  eurekensis  Triplesia  calcifera  in 
the  lower  portion,  followed  higher  up  by  the  siiine  forms  as  found 
at  Eureka  east  of  the  Jackson  mine  and  east  of  Hamburg  Eidge. 


192  GEOLOGY  OF  THE  EUKEKA  DiSTEICT. 

HAMBURG. 

Feet. 

10.  Dark  bluish  black  limestone,  carrying  Hamburg  limestone  fossils 800 

7,800 

Divided  according  to  the  epochs  adopted  at  Eureka  we  have : 

Lone  Mountain  limestone 1, 450 

Eureka  quartzite 350 

Pogonip  limestone 5, 200 

Hamburg  limestone 800 

The  Hamburg  limestone  yielded  the  following  species: 

Protospongia  sp.!  Conocephalites  sp.f 

Lingulepis  niinuta.  Crepicephalus  nitidus. 

Orthis  sp.f  Crepicephalus  unisulcatus. 

Agnostus  bidens.  Chariocephalus  tumifrons. 

Agnostus  communis.  Illaenurus  sp.  f 

While  at  White  Pine  the  relationship  between  the  Cambrian  and 
Silurian  is  well  shown,  the  Devonian  has  not  been  recognized  directly  over- 
lying the  Lone  Mountain  Silurian.  Between  Pogonip  Mountain  and  the 
next  ridge  to  the  eastward  a  displacement  brings  up  the  Nevada  limestone, 
forming  the  massive  beds  of  Mount  Argyle  and  Treasure  Peak.1  This 
limestone  is  here  overlain  by  the  black  argillaceous  shale,  which  passes 
into  sandstone,  followed  by  Carboniferous  limestone.  The  black  shale  is 
the  counterpart  of  the  corresponding  terrane  at  Eureka,  a  comparison  of 
the  two  sections  showing  the  greatest  resemblance.  The  coarse  yellow 
sandstone  above  seems  to  be  the  equivalent  of  the  Diamond  Peak  quartz- 
ite, although  here  at  White  Pine  it  is  represented  by  only  a  few  hundred 
feet,  while  the  black  shale  attains  a  development  of  1,000  feet.  From  the 
Nevada  limestone  there  has  been  collected  an  abundant  fauna  characteristic 
of  the  middle  and  upper  beds.  It  was  for  the  most  part  obtained  by  the 

'U.  8.   Geological  Exploration   of  the   Fortieth     Parallel,  vol.    3,    Mining  Industry,  p.  409,    and 

accompanying  atlas  sheet  14. 


DEVONIAN  AT  WHITE  PINE.  193 

writer  from  Mount  Argyle  and  Treasure  Peak,  and  represents  thirty -three 
genera  and  forty-nine  species,  as  follows: 

Cyathopliyllum  sp.  ?  Eetzia  radialis. 

Feuestella  (2  sp.  ?)  Atrypa  reticularis. 

Thainniscus  sp. ?  Bhynchonella  duplicata. 

Liugula  alba-pinensis.  Ehynchonella  emmonsi. 

Discina  lodensis.  Ehynchonella  occidens. 

Chonetes  sp.?  Ehynchonella  (L)  quadricostata. 

Strophodonta  canace.  Cryptouella  circula. 

Strophodouta  inequiradiata.  Pentainerus  lotis. 

Strophodonta  sp.?  Terebratula  sp.? 

Orthis  macfarlani.  Aviculopecten  catactns. 

Orthis  impressa.  Pterinopecten  sp. ? 

Productus  hirsntiforme.  Lunulicardium  fragosmn. 

Productus  subaculeatus.  Cardiomorpha  missonriensis. 

Productus  sp.  ?  Nuculites  triangulus. 

Spirifera  alba-pineusis.  Paracyclas  peroccidens. 

Spirifera  disjuncta.  Gonocardium  sp.? 

Spirifera  engelmaniii.  Platyostoma  sp.  ? 

Spirifera  pinonensis.  Euomphalvis  laxus. 

Spirifera  strigosus.  Euomphalus  sp.? 

Spirifera  subuuibona.  Loxonema  sp.  ? 

Spirifera  sp.  ?  Platyschisma  sp.  ? 

Cyrtina  davidsoni.  Bellerophon  neleus. 
Ambocffilia  unibonata. 

A  more  characteristic  White  Pine  fauna  is  preserved  in  the  black  shale 
than  has  yet  been  obtained  in  the  corresponding  beds  at  Eureka,  and  a 
belt  of  intercalated  limestone  in  the  shale  similar  to  that  found  east  of 
Sugar  Loaf  at  Eureka  bears  equal  evidence  of  its  Devonian  age.  Here 
the  limestone  appears  as  a  lenticular  body  in  the  shale,  with  beds  identical 
in  composition  both  above  and  below.  While  there  is  much  in  the  group- 
ing of  forms  foreshadowing  the  Carboniferous,  the  shales  maintain  their 
Devonian  aspect  by  carrying  certain  characteristic  species  up  nearly  to  the 
top  of  the  series,  and  in  this  respect  resemble  the  black  shales  found  at 
Hays  Canyon  west  of  Newark  Mountain. 
MON  xx 13 


194  GEOLOGY  OF  THE  EUKEKA  DISTRICT. 

From  Applegarth  Canyon  the  White  Pine  shales  yielded  the  following 
species: 

Cyathophyllurn  sp.  ?  Athyris  (of  type  of  A.  plano-sulcata). 

Penestella  (2  sp.  ?)  Bhynchouella  (L)  quadricostata. 

Thamniscus  ?  sp.  1  Aviculopecten  catactus. 

Lingula  alba-pinensis.  Nuculites  triangulus. 

Discina  lodensis.  Cardiomorpha  missouriensis. 
Chonetes  (of  type  of  C.  illinoisensis).      Lunulicardium  fragosum. 

Productus  hirsutiforine.  Hyolithes  sp.  ? 

Productus  subaculeatus.  Pleurotomaria  sp. ! 
Productus  (of  type  of  P.   semireticu-  Goniatites  kingii. 

latus).  Goniatites  sp.l 

Spiriferina  cristata.  Prcetus  sp.  f 

Ambocffilia  umbonata.  Cytoceras  cessator. 
Eetzia  radialis. 

With  the  exception  of  some  indeterminable  fragments  of  crinoid  col- 
umns and  a  few  impressions  of  stems  and  twigs,  the  sandstones  have  yielded 
no  life.  The  few  vegetable  remains,  however,  are  important,  as  they  are 
of  rare  occurrence  in  Paleozoic  sandstones  of  Nevada.  The  Carboniferous 
limestones  overlying  this  belt  of  sandstones  have  been  but  little  studied 
since  the  explorations  of  the  fortieth  parallel,  and  no  additional  material 
throwing  light  upon  the  life  of  the  period  has  been  obtained. 

Silurian  and  Highland  Range.— In  the  Highland  Range  the  Silurian  rocks  have 
not  been  as  carefully  studied  as  the  Cambrian.  Indeed,  it  is  by  no  means 
certain  that  in  the  area  covered  or  in  the  exposure  of  beds  that  the  Silurian 
is  as  well  represented  as  in  a  number  of  other  ranges,  although,  as  has  been 
already  shown,  the  Cambrian  compares  favorably  with  the  same  epoch  in 
the  Eureka  Mountains.  Both  the  Pogonip  and  Eureka  quartzite,  however, 
are  well  exposed  on  the  west  side  of  the  range  in  a  hill  just  north  of  the 
road  leading  from  Bennett  Spring  to  Hyko,  where  the  fauna  in  the  lime- 
stone immediately  below  the  quartzite  is  so  characteristic  that  both  forma- 


FOSSIL  BUTTE.  195 

tions  are  readily  determined.     At  this  locality,  in  beds  below  the  Eureka 
quartzite,  Mr.  Walcott  made  the  following  collection: 

( )rthis  perveta.  Subulites  sp.  ? 

Orthis  tricenaria.  Orthoceras  sp  ? 

Orthis  pogonipensis.  Ortlioceras  (Aiiiiulated  species). 

Strophornena  fontinalis.  Leperditia  bivia. 

Modiolopsis  occidens.  Ceraurus  sp. ! 

Modiolopsis  pogonipensis.  Illamus  crassicauda. 

Raphistoma  acuta.  Bathyurus  pogonipensis. 

Murchisonia,  2  sp.*  Pleurotomaria  lonensis. 

Fossil  Butte.— At  Fossil  Butte,  10  miles  north  of  Hyko,  on  the  east  side 
of  Pahranagat  Valley,  the  Pogonip  is  again  seen  overlain  by  the  Eureka 
quartzite.  The  butte  stands  out  as  an  elevated  ridge,  but  presenting  an 
exactly  similar  succession  of  strata  as  seen  along  the  east  side  of  Prospect 
Mountain,  Pogonip  Ridge,  and  the  western  base  of  Lone  Mountain.  In  the 
limestone  occur  the  following  species: 

Receptaculites  mammillaris.  Metoptoiua  phillipsi. 

Orthis  tricenaria.  Ecculiomphalus,  like  E.  distans. 

Strophornena  fontinalis.  Orthoceras  rnulticameratum. 

Triplesia?  sp.?  Endoceras  multitubulatum. 

Leperditia  bivia.  Modiolopsis  occidens. 

Maclurea  subannulata.  Modiolopsis  pogonipensis. 

Maclurea,  2  sp.,  undet.  Illaenus  crassicaiida. 

Taken  together  these  two  groups  from  Bennett  Spring  and  Fossil 
Butte  carry  the  more  marked  fauna  of  the  Upper  Pogonip.  Overlying  the 
quartzite  occur  some  light  gray  limestones,  without  organic  remains,  but 
resembling  the  Lone  Mountain  beds.  Along  the  east  side  of  Pahranagat 
Valley  limestone  ridges  extend  for  several  miles.  The  beds  have  been 
much  disturbed  and  have  undergone  considerable  faulting,  preventing  accu- 
rate sections^  but  it  is  estimated  that  there  are  from  2,000  to  3,000  feet  of 
limestones  exposed.  They  are  more  or  less  siliceous,  weathering  reddish 
brown  and  brownish  gray.  The  lower  members  may  possibly  belong  to  the 
Lone  Mountain  series.  Near  Hyko  there  is  an  exposure  of  shaly  limestone, 
overlain  by  massive  beds  of  dark  arenaceous  limestone,  carrying  a  Devonian 


196  GEOLOGY  OF  THE  EUKEKA  DISTRICT. 

fauua.     The  specimens,  although  poorly  preserved,  allowed  of  the  following 
determinations: 

Stromatopora  sp.  t  Modiomorpha  sp. 

Spirifera  sp. !  Holopea  sp.  ? 

Atrypa  reticularis.  Euomphalus  (P)  laxus. 
Pentamerus  lotus. 

Pahranagat  Range.— On  the  west  side  of  the  valley  the  Pahrauagat  Range 
forms  a  long,  continuous  ridge,  for  the  most  part  made  up  of  sedimentary 
strata  faulted  and  broken  into  massive  blocks  by  outbursts  of  acidic  lavas. 
Quartz  Peak,  the  culminating  point,  affords  a  fine  exposure  of  Silurian 
strata,  with  much  the  same  series  of  beds  as  seen  in  the  central  part  of  the 
State,  with  this  exception  that  neither  the  Upper  Pogonip  at  the  base  nor  the 
Niagara  at  the  summit  are  represented  in  their  full  development.  Upon  the 
south  side  of  the  peak,  extending  from  the  base  to  the  summit,  there  is  an 
unbroken  exposure  of  strata  2,000  feet  in  thickness,  striking  N.  30°  E., 
with  an  average  dip  of  20°  N.  The  summit  of  the  peak  is  formed  of  the 
Niagara  limestone.  The  section,  with  the  accompanying  fossils,  is,  accord- 
ing to  Mr.  Walcott,  as  follows: 

LONE   MOUNTAIN — NIAGARA. 

Feet. 

1.  Massive  bedded  gray  siliceous  limestone,  with  occasional  layers  of  sand- 

stone and  chert 535 

LONE  MOUNTAIN — TEENTON. 

2.  Massive  bedded  dark  siliceous  limestone,  with  a  stratum  30  feet  thick, 

almost  made  up  of  a  species  of  Pentamerus  like  P.  galeatus.    These 

occur  not  far  above  No.  3 335 

3.  Bluish  black  and  bluish  gray  thin  bedded  limestone,  with  numerous  fossils .  30 

Zaphrentis,  sp.  ?,  Bryozoa,  3  sp.,  Streptorhynchus  filitexta,  Orthis  tes- 
tudinaria. 

4.  Massive  bedded  dark  iron -gray  siliceous  limestone : 150 

EUREKA. 

5.  Hard,  vitreous  white  quartzite,  becoming  tinged  with  a  reddish  color  toward 

the  base 400 


PAHBANAGAT  KANGE.  197 

POGONIP. 

Feet. 
6.  Evenly  bedded  layers  of  a  dark  bluish  black  and  bluish  gray  limestone,  thin 

layers  making  more  massive  beds  that  break  up  on  exposure  to  the 

influence  of  the  atmosphere 150 

Beceptaculites  mammillaris,  Orthis  pogonipensin,  Orthis  tricenaria, 
Porambonites  obscurutt,  Bellerophon,  spJ,  Hyolites,  sp.  undet.,  Endo- 
ceras  multitubulatum,  Leperditia  bivia,  Illcenus  crassicauda. 

1.  Thinner  bedded  bluish  gray  limestone  that  is  shaly  in  places 400 

Fossils  numerous. 

8.  Massive  bedded  gray  limestone,  in  layers  from  1  to  4  feet  in  thickness  ....          200 
Orthis,  Murchisonia,  and  Orthoceras  are  seen  in  the  lower  layers,  and 
Beceptaculites  mammillaris  and  B.  elongata  150  feet  higher  up. 


2,200 
This  section  gives  for  the  different  horizons  as  follows: 

Niagara 535 

Trenton 515 

Eureka 400 

Pogonip 750 

On  the  northeast  slope  of  Quartz  Peak  there  is  a  heavy  mass  of  light 
gray  siliceous  limestone,  roughly  estimated  at  1,000  feet,  without  fossils, 
which,  by  its  stratigraphical  position  and  lithological  habit  is  easily 
referred  to  the  upper  beds  of  the  Lone  Mountain,  a  continuation  of  the 
beds  upon  the  summit  of  the  peak  as  given  in  the  section.  To  the  south  of 
Quartz  Peak  occurs  a  great  development  of  limestone.  A  section  across 
the  beds  is  of  special  interest,  owing  to  the  thickness  of  the  limestones 
from  the  Lone  Mountain  to  the  Carboniferous,  which  is  unbroken  by  the 
presence  either  of  Diamond  Peak  quartzite  or  White  Pine  shale,  as  in  both 
the  Eureka  and  White  Pine  sections.  The  section  is  as  follows: 

CARBONIFEROUS. 

Feet. 

1.  Siliceous  limestone,  sandstone,  and  quartzite 500 

2.  Cherty  siliceous  limestone 250 

3.  Shaly  limestone  in  massive  layers 55 

4.  Massive  bedded  gray  limestone,  hard  and  compact ;  it  passes  into  gran- 

ular dark  gray  limestone  and  then  into  more  thinly  bedded  bluish 

black  limestone 1, 260 


198  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

Feet, 

5.  Bluish  black  limestone  in  tbiu  layers,  overlain  by  shaly  limestone,  with 

intercalated  beds  of  bluish  black  limestone.  The  limestone  in  layers 
gradually  replaces  the  shaly  limestone  until  the  latter  disappears 
from  the  section.  Upper  beds  characterized  by  Lower  Carboniferous 

fauna.     (List  follows  the  section.) 390 

At  95  feet  from  the  top,  Spirifera  lineata  and  Spirifera  cristata  were 
observed. 

DEVONIAN. 

At  140  feet  from  the  top,  numerous  fragments  of  crinoids,  crinoidal 
columns  were  seen  for  the  last  time,  and  Atrypa  reticularis  began  to 
appear. 

At  350  feet  from  the  top,  buff  shaly  limestone  predominates  and  a  more 
evenly  grained,  smoother,  harder  limestone  begins  to  appear  as  thin 
layers  in  the  shaly  limestones.  Fossils  few  in  number  and  badly  pre- 
served. 

6.  Hard,  compact,  yellowish  sandstone  in  thin  layers 25 

7.  Calcareous  sandstone  overlain  by  arenaceous  limestone,  and  above  that 

bluish  gray  thin  bedded  limestone 175 

8.  Gray  siliceous  limestone  with  shaly  limestone  partings  and  bands  of  bluish 

black  limestone 340 

At  125  feet  from  the  top,  a  band  of  bluish  black  thinly  bedded  limestone 

carries  numerous  fossils  of  the  Upper  Devonian  age.     (List  follows 

the  section.) 
At  240  feet  from  the  top,  Stromatopora  and  coralline  markings  appear. 

9.  Hard  buff  colored  sandstone 25 

10.  This  is  almost  a  repetition  of  No.  8,  the  three  grading  into  each  other  in 

places 225 

Seventy-five  feet  down  from  the  top,  Stromatopora  and  a  small  slender 
coralline  stem  crowd  the  darker  siliceous  layers. 

11.  Light  gray  siliceous  limestone,  almost  a  sandstone  in  places,  passing  up 

into  a  dark  siliceous  limestone,  and  then  into  thinner  bedded  bluish 

black  and  bluish  limestone 1, 920 

The  upper  layers  contain  Strophomena  pvrplana,  Atrypa  reticularis,  Cyr- 
tina,  sp.f  Pleurotomaria. 

12.  Gray  quartzitic  sandstone  in  massive  layers 100 

13.  Gray  siliceous  limestone 110 

14.  Quartzitic  ferruginous  sandstone 85 

15.  Light  gray  and  dirty  brown  siliceous  limestone  in  alternating  bands  of 

color,  of  varying  degrees  of  hardness.  The  siliceous  and  calcareous 
matter  varies  considerably  in  the  different  layers.  Toward  the  lower 
portion  many  layers  are  almost  made  up  of  a  species  of  Stromatopora 
and  slender  stems  of  a  branching  coral  one-eighth  to  one-fourth  of  an 
inch  in  diameter 2, 100 


DEVONIAN  AND  CARBONIFEROUS  FAUNA.  199 

SILURIAN. 

Feet. 
16.  Light  gray  siliceous  limestone 1,000 


8,560 

In  the  bluish  black  limestone  at  the  top  of  No.  5  the  following  Lower 
Carboniferous  fauna  comes  in  : 

Amplexus,  sp.  ?  Spirifera  (M)  lineata. 

Syringopora,  sp.  ?  Cyrtina,  sp?. 

Acervularia  pentagona.  Athyris  subquadrata. 

Fenestella,  sp.?  Rhynchonella. 

Chonetes,  sp.  I  Terebratula,  sp.  ? 

Chonetes  granulifera.  Platyceras,  sp.? 

Chonetes,  sp.?  Bellerophon,  sp.? 

Productus  nebrascensis.  Euomphalus,  sp.  ? 

Productns  punctatus.  Euomphalus  laxus. 

Productus  tenuicostatus.  Euomphalus  (Straparollus)  ophinea. 

Productus  semireticulatus.  Straparollus,  sp.  ? 

Productus,  sp!  Holopea,  sp.? 

Productus,  sp  ?  Loxonema. 

Orthis  resupinata.  Loxouema. 

Streptorhynchus  cremstria.  Pleurotomaria,  sp.  ? 

Syringothyris  cuspidatus.  Pleurotomaria,  sp.  ? 

Spirifera  pinguis.  Edmondia,  2  sp.  ? 

Spirifera  pulchra.  Leperditia,  sp.  ? 

Spirifera  striata.  Prcetus  peroccidens. 

The  following  Devonian  fauna  was  collected  from  No.  8 : 

Lingula  (like  L.  ligea).  Rhynchonella  sinuata. 

Orthis  impressa.  Athyris  ?  sp.  ? 

Productus  shumardianus.  Pentamerus  lotus. 

Productus  (like  P.  lachryinosa).  Modiomorpha,  sp? 

Strophodonta,  sp.  ?  Euomphalus,  sp.  ? 

Spirifera,  sp.?  Platyostoma  lineata? 

Nucleospira  concinna.  Orthoceras,  sp.  ? 

Cyrtina  hamiltonensis.  Orthoceras,  sp.  ? 
Amboco3lia  (like  young  of  A.  umbonata).         Leperditia,  sp:? 
Rhynchonella  duplicata. 

In  this  section  we  have  about  8,000  feet  of  nearly  continuous  lime- 
stone strata,  broken  occasionally  by  thin  beds  of   yellow  sandstone,  the 


200  GEOLOGY  OF  THE  EUREKA  DISTEICT. 

heaviest  not  over  100  feet  in  thickness.  It  is  not  singular  that  in  a  massive 
bed  of  limestone  it  becomes  a  difficult  matter  to  divide  the  Silurian, 
Devonian,  and  Carboniferous  with  any  degree  of  precision.  Between  the 
Silurian  and  Devonian  at  the  base  of  the  section  it  is  impossible  to  draw 
any  line  of  demarcation.  It  is  safe,  provisionally,  to  place  1,000  feet  of  the 
light  gray  limestone  in  the  Lone  Mountain  period,  leaving  strata  carry- 
ing Stromatopora  and  branching  corals  included  in  the  Devonian.  In  the 
390  feet  of  bluish  black  limestones  (No.  5  of  the  section)  a  marked 
Devonian  fauna  occurs  at  the  base  and  Lower  Carboniferous  fauna  at  the 
summit,  without  any  change  in  the  lithological  character  of  the  beds.  Pro- 
visionally the  line  is  drawn  so  as  to  include  in  the  Carboniferous  all  beds 
carrying  Spirifera  lineata  and  Spirifera  cristata,  and  leaving  Atrypa  reticularis 
in  the  Devonian.  By  this  division  we  have  the  following  thicknesses  for 
the  different  periods: 

Feet. 

Carboniferous 2, 160 

Devonian 5, 400 

Silurian 1, 000 

Pinon  Range.— To  these  sections  south  and  southeast  of  Eureka  may  be 
added  still  another,  constructed  across  Pinon  Range  about  60  miles  north- 
ward. This  range  is  a  long,  narrow  ridge,  stretching  from  the  Humboldt 
River  southward  until  it  joins  the  Eureka  Mountains  at  The  Gate,  the 
southern  end  of  the  range  coming  within  the  area  of  this  survey. 

The  Pinon  Range  attains  its  greatest  elevation  just  south  of  the  Hum- 
boldt, where  the  best  continuous  sections  of  the  Lower  Paleozoic  rocks 
occur.  The  range  was  crossed  at  several  points  by  the  geologists  of  the 
Fortieth  Parallel  Exploration,  the  Devonian  rocks  being  traced  by  the  writer 
for  nearly  their  entire  length  from  The  Gate  to  the  Humboldt  River.  At 
the  request  of  the  writer,  Mr.  Walcott  visited  the  northern  end  for  the  pur- 
pose of  a  comparative  study  of  the  section  exposed  at  Ravens  Nest  with 
the  corresponding  rocks  at  Eureka.  Here  the  beds  strike  obliquely  across 
the  trend  of  the  range  from  the  northwest  base  of  Ravens  Nest  to  the  base 
of  Pinto  Peak,  the  course  of  the  range  being  approximately  north  and 
south. 


PlSfON  RANGE. 
The  following  ideal  section  was  made  by  Mr.  Walcott: 


201 


fcF^ 
fc-^ 


J^tmestone-. 


JEiiretta. 


limestone.  "Limestone. 

fv\.  4. — Section  across  Pinon  Range. 


Carboniferous 


Across  the  range  from  west  to  east  along  the  line  of  the  section  a  dark 
blue  limestone,  carrying  a  few  fossils  of  the  Lower  Devonian,  rises  above 
the  plain.  Beyond  the  limestone  a  sharp  oblique  fault  brings  up  a  broad 
mass  of  quartzite,  conglomerates,  and  black  siliceous  pebbles.  These,  in 
turn,  are  conformably  underlain  by  blue  limestone,  from  which  a  sufficient 
fauna  was  secured  to  identify  Upper  Devonian  beds.  The  limestones  are 
again  cut  off  by  a  profound  fault,  apparently  along  a  line  of  an  anticlinal 
axis.  To  the  west  of  this  fault  the  beds  all  dip  westward,  but  beyond  this 
point  present  an  easterly  dip,  at  least  as  far  as  the  base  of  Pinto  Peak. 
Directly  eastward  of  the  fault  a  dark  ferruginous  quartzite  stands  out 
prominently,  followed  by  light  gray  siliceous  limestones,  the  age  of  which 
is  determined  by  the  presence  of  Halysites.  The  beds  gradually  assume  the 
habit  of  the  Devonian  and  carry  a  fauna  sufficiently  characteristic  to  estab- 
lish the  horizon  of  the  Lower  Devonian,  and  still  higher  up  in  the  series 
yielded  Upper  Devonian  species.  Overlying  the  limestones  occurs  a  great 
thickness  of  quartzites  and  sandstones,  with  occasional  argillaceous  bed. 
Near  the  junction  of  the  Devonian  limestones  with  the  overlying  quartzites 
the  beds  upon  both  sides  of  the  anticline  are  identical,  showing  the  corre- 
sponding horizons  without  the  evidence  of  the  fauna.  If  the  White  Pine 
shale  of  Eureka  is  at  all  represented  in  the  Pinon  Range  it  is  found  in  the 
argillaceous  and  finely  siliceous  beds  immediately  overlying  the  Devonian 
limestones  on  both  sides  of  the  fault.  These  beds  resemble  those  observed 
at  the  same  horizon  at  The  Gate,  already  described,  and  the  continuance 
northward  of  similar  sediments  is  not  without  interest,  especially  when 
taken  in  connection  with  the  great  thickness  of  White  Pine  shale  to  the 
southeast  as  developed  in  the  Diamond  Range,  Cliff  Hills  and  White  Pine. 


202  GEOLOGY  OP  THE  EUREKA  DISTRICT. 

Roughly  estimated,  the  thicknesses  of  the  beds  in  the  above  section 
are  as  follows: 

Feet. 

Eureka  quartzite 400 

Silurian  and  Devonian  limestones 5, 500 

Carboniferous  quartzites  and  sandstones 7, 000 

Tucubit  Mountains.— In  the  Tucubit  or  Wild  Cat  Mountains,  north  of  the 
Humboldt  River,  occurs  a  long  stretch  of  massive  limestones  that  have  evi- 
dently undergone  much  faulting  and  disturbance.  In  a  black  calcareous 
shale  belt  overlying  yellow  calcareous  shales  there  were  found  Atrypa  reti- 
cularis,  Spirifera  vanuxemi  and  Orthis  multistriata,  and  other  species,  show- 
ing the  extension  of  Devonian  beds  into  the  northern  part  of  the  State. 
These  limestones  in  the  Tucubit  Mountains  have  been  estimated  by  Mr.  S. 
F.  Emmons  as  from  4,000  to  5,000  feet  in  thickness.1 

Unconformity  in  the  Silurian.— In  the  descriptive  chapters  references  have  fre- 
quently been  made  to  an  unconformity  existing  in  the  Silurian,  between 
the  Eureka  quartzite  and  Lone  Mountain  limestone.  This  unconformity  is 
also  indicated  in  the  accompanying  atlas  sheets,  where  different  horizons  of 
the  Lone  Mountain  and  Nevada  limestones  are  seen  to  rest  directly  upon 
the  underlying  quartzite.  The  Lone  Mountain  beds  may  frequently  be 
seen  to  wedge  out  in  places ;  in  others,  Devonian  beds,  determined  as  such 
by  their  associated  fossils,  come  down  nearly  if  not  quite  to  the  top  of  the 
Eureka  quartzite.  In  only  one  locality  along  the  southern  base  of  Comb's 
Mountain  do  the  beds  show  the  lithological  characters  or  the  fauna  of  the 
Trenton  limestone,  but  here  they  are  overlain  by  a  broad  development  of 
the  Niagara,  the  upper  member  of  the  Lone  Mountain  epoch,  before  the 
coming  in  of  the  Devonian.  Evidences  of  uncomformity  by  erosion  have 
been  recognized  in  a  few  localities,  but  the  nature  of  the  quartzite  is  such 
that  they  could  hardly  be  expected  to  be  conclusive,  the  amount  of 
erosion  being  slight.  In  most  instances  in  the  more  elevated  regions  where 
the  Eureka  quartzite  lies  horizontally  the  overlying  limestones  have  been 
eroded,  but  this,  however,  is  not  the  case  on  the  -plateau  in  the  region  of 
Grays  Peak,  where  isolated  patches  of  limestone  occupy  depressed  areas 
and  shallow  basins  in  the  undulating  surface  of  quartzite. 

1 II.  S.  Geological  Exploration  of  the  Fortieth  Parallel,  vol.  2.  Descriptive  Geology,  p.  524. 


MOVEMENT  IN  CARBONIFEROUS.  203 

Movement  in  Carboniferous.— In  the  chapter  devoted  to  the  Upper  Coal- 
measures  descriptions  have  been  given  of  the  coarse  conglomerates  inter- 
bedded  in  the  limestone,  containing  siliceous  pebbles  and  worn  fragments 
of  limestone  carrying  Coal-measure  fossils,  evidently  derived  from  neigh- 
boring land  areas  undergoing  denudation.  Many  of  the  siliceous  pebbles 
have  the  appearance  of  coming  from  the  Weber  conglomerate,  but  this 
can  not  be  positively  stated.  The  Productus  semireticulatus  and  other 
species  found  in  these  rolled  fragments  may  have  been  derived  from  either 
the  Upper  or  Lower  Coal-measures.  The  change  in  sediment  from  lime- 
stone to  conglomerate  is  abrupt,  and  no  indications  of  erosion  in  the  under- 
lying rocks  were  observed.  The  evidence  of  movement  rests  wholly  upon 
the  lithological  character  of  the  material  forming  the  conglomerate,  which 
was  in  turn  covered  by  deposits  in  every  way  similar  to  the  limestone 
below  it.  It  seems  evident  that  fragments  of  fossil-bearing  limestone  could 
not  have  withstood  the  disintegrating  action  of  water  for  any  great  length 
of  time  or  have  been  transported  for  any  great  distance. 

Distribution  of  Upper  Silurian  and  Devonian.— Over     large      areas   of     the     Great 

Basin,  Upper  Silurian  and  Devonian  sediments  are  either  wanting  or  have 
not  as  yet  been  recognized.  Several  longitudinal  ranges  have  been  described 
in  their  geological  structure  as  tilted  blocks  formed  either  exclusively  of 
Carboniferous  rocks,  or  else  made  up  of  the  Pogonip  of  the  Lower  Silu- 
rian overlain  conformably  by  Coal-measure  limestones,  the  intervening 
horizons,  which  at  Eureka  are  known  to  measure  over  12,000  feet  in  thick- 
ness, being  entirely  unrepresented.  Sediments  of  Upper  Silurian  and 
Devonian  age,  while  they  occupy  limited  areas,  nevertheless  play  a  most 
important  part  in  the  ranges  which  rib  the  central  portion  of  Nevada.  To 
the  north  of  the  Humboldt  River,  as  already  pointed  out,  the  Nevada  lime- 
stones have  been  recognized  in  the  Tucubit  Mountains  ;  they  form  the 
greater  part  of  the  Roberts  Peak  Mountains  west  of  the  Pifion  Range, 
where  they  probably  overlie  a  considerable  but  unknown  thickness  of  Lone 
Mountain  Silurian,  and  the  writer  has  traced  the  Devonian  beds  all  along 
the  Pinon  Range,  connecting  them  with  the  grand  exposures  of  the  Eureka 
district.  At  Ravens  Nest,  at  the  northern  end  of  this  latter  range,  that 
portion  of  the  Eureka  series  of  beds  lying  between  the  Silurian  quartzitr 
and  the  Coal-measures  is  strikingly  reproduced,  the  structure  being  a  faulted 


204  GEOLOGY  OF  THE  EUKEKA  DISTRICT. 

anticline  with  corresponding  beds  upon  both  sides  of  the  axial  plane. 
Between  the  ancient  shore-line  to  the  west  and  the  Humboldt  Range 
on  the  east,  there  appears  to  have  been  a  deep  meridional  trough-like 
depression  in  which  all  the  beds  from  the  Eureka  quartzite  up  to  the  top  of 
the  Devonian  were  deposited.  How  far  north Avard  they  can  be  traced  con- 
tinuously is  still  a  matter  of  conjecture,  but  we  know  that  the  Devonian 
beds  occupy  large  areas  along  the  valley  of  the  Mackenzie  River.  To  the 
south  this  narrow  channel  or  trough  apparently  widens  out  into  a  broad 
bay  or  open  sea. 

To  the  southeast  of  Eureka,  in  the  White  Pine  Mountains,  the  Eureka 
quartzite,  and  both  the  Trenton  and  Niagara  members  of  the  Lone  Moun- 
tain epoch  are  well  developed  on  the  northeast  end  of  Pogonip  Ridge,  and 
the  Devonian  on  Treasure  and  Babylon  hills.  The  sequence  of  strata, 
together  with  the  associated  fauna  from  the  base  of  the  Pogonip  to  the 
Diamond  Peak  quartzite,  may  be  easily  correlated  in  the  two  localities. 
Still  farther  southward,  on  the  east  side  of  Pahranagat  Valley,  both  the 
Upper  Silurian  and  Devonian  are  exposed  in  a  great  thickness  of  limestones 
bordering  the  valley.  In  the  uplifted  block  at  Quartz  Peak,  in  the  Pahran- 
agat Range,  we  have  Pugonip,  Eureka,  Trenton,  and  Niagara  all  well 
exposed,  but  neither  the  upper  nor  lower  horizon  is  shown  in  its  full  devel- 
opment. 

Special  mention  should  be  made  of  the  grand  exposure  of  limestone 
found  south  of  Quartz  Peak.  Here  we  have  over  8,000  feet  of  conform- 
able beds  starting  in  with  the  Niagara  at  the  base,  passing  through  a  great 
thickness  of  Devonian,  and  continuing  on  up  into  beds  characterized  by 
a  rich  fauna  of  the  Lower  Carboniferous.  In  this  section  the  White 
Pine  shale  and  the  Diamond  Peak  quartzite  are  wholly  wanting.  While 
this  series  of  beds  shows  in  some  respects  the  closest  resemblance  to  the 
Eureka  section,  it  is  also  significant  as  indicating  an  equally  strong  resem- 
blance to  the  massive  body  of  Wasatch  limestone  carrying  Silurian,  Devo- 
nian, and  Lower  Coal-measure  limestones  without  intervening  siliceous 
belts  of  any  considerable  thickness. 

Such  exposures  on  a  grand  scale  are  sufficient  to  show  a  very  great 
development  of  Silurian  and  Devonian  rocks  stretching  for  long  distances 
from  southeastern  Nevada  well  up  toward  the  northern  part  of  the  State, 


DISTRIBUTION  OF  THE  PALEOZOIC.  205 

if  not  far  beyond.  Throughout  this  entire  distance,  wherever  they  have 
been  studied,  these  limestones  maintain  a  great  thickness  of  strata.  At  the 
southern  end  of  this  belt  near  Quartz  Peak,  the  Silurian  and  Devonian 
limestones  are  estimated  at  G,40()  feet,  and  at  Ravens  Nest,  just  south  of 
the  Humboldt  River,  the  estimate  gives  5,500  feet,  while  at  Eureka  they 
present  even  a  greater  thickness. 

As  yet  we  know  but  little  about  the  occurrences  and  distribution  of  the 
Diamond  Peak  quartzite.  According  to  the  section  at  Quartz  Peak  it  is 
wholly  wanting.  In  the  Diamond  Range  at  Eureka,  it  attains  a  thickness 
of  3,000  feet ;  on  the  opposite  side  of  the  valley  in  the  Pinon  Range  it  can 
not  measure  less,  and  at  Ravens  Nest  it  attains  a  development  of  7,000 
feet.  In  the  northern  part  of  Nevada  we  see  an  enormous  development  of 
arenaceous  beds  separating  a  Devonian  from  a  Carboniferous  fayua.  This 
material  thins  out  to  the  south,  and  in  place  of  it  we  find  a  continuous 
limestone  body  extending  all  the  way  from  the  Eureka  quartzite  well 
up  into  the  Carboniferous,  without  any  well  defined  intervening  sili- 
ceous horizon.  Too  few  observations  have  "been  made  to  determine 
the  geological  history  or  geographical  distribution  of  the  Upper 
Silurian  and  Devonian  rocks.  As  to  the  character  of  their  sedi- 
mentation eastward,  the  thickening  or  thinning  out  of  strata,  or  their 
lithological  transitions,  we  know  but  little.  It  is  a  most  significant  fact, 
and  one  by  no  means  easy  to  explain,  that  the  entire  series  of  beds  included 
within  the  second  period  in  which  the  Paleozoic  rocks  of  Eureka  have  been 
classed,  based  upon  their  physical  history,  should  apparently  be  wanting 
over  such  large  areas  in  Utah  and  eastern  Nevada.  In  other  localities,  while 
they  may  not  be  wholly  wanting,  the}'  appear  to  be  represented  by  thin 
beds  of  Lone  Mountain  strata,  identified  by  a  stray  Halysites,  and  the  Devo- 
nian by  an  occasional  Atri/pa. 

Another  interesting  fact  as  regards  the  position  of  these  rocks  is  this : 
Notwithstanding  the  enormous  thickness  of  the  Upper  Silurian  and  Devo- 
nian beds  at  Eureka,  the  same  relative  position  which  has  been  observed  in 
so  maiiv  plac.es  elsewhere,  with  the  Coal-measures  resting  upon  the  under- 
lying Pogonip,  may  be  seen  here,  the  Upper  Silurian  and  Devonian  being 
absent.  The  occurrence  of  the  two  limestone  bodies  lying  in  juxtaposition 
may  be  seen  all  along  the  east  base  of  Prospect  Ridge,  where  the  Iloosac 


200 


GEOLOGY  OF  THE  EUREKA  DISTRICT. 


fault  brings  the  Lower  Coal-measures  up  against  the  Pogonip,  organic 
remains  characteristic  of  both  epochs  being  found  within  a  few  hundred 
yards  of  each  other,  with  the  intervening  space  occupied  mainly  by 
igneous  extrusions  along  the  fault-line.  Here,  however,  they  have  been 
brought  into  their  present  position  by  profound  orographic  displacement. 

The  Wasatch  and  Kanab  Sections.— The  Wasatch  Range,  which  shuts  in  the 
Great  Basin  on  the  east,  combines,  in  a  marked  manner,  many  of  the  geo- 
logical characters  of  both  the  Rocky  Mountains  and  the  Basin  ranges.  In 
structure,  however,  it  is  closely  related  in  its  essential  features  to  the  ranges 
of  Utah  and  Nevada.  There  is  exposed  in  the  range  a  very  remarkable 
section  of  conformable  beds,  extending  through  30,000  feet  of  sediments 
and  exhibiting  nearly  every  geological  period  from  Lower  Cambrian  to 
Permian.  ^  For  the  purpose  of  comparison  a  section  constructed  by  the 
Geological  Exploration  of  the  Fortieth  Parallel  is  reproduced,  as  it  shows 
not  only  certain  resemblances,  but  also  striking  differences  in  the  sequence 
beds  from  the  section  as  exposed  at  Eureka : 

Wasatch  section,   Utah:  30,000  feet;  conformable. 


PERMIAN,  650  feet  . 


CARBONIFEROUS, 
14,000  feet 


DEVONIAN,  2,400  feet.  < 

SILURIAN,  1,000  feet  ... 
CAMBRIAN,  12,000  feet  . 


Permian 

Upper  Coal-measure  limestone . 

Weber  quartzite 

Lower  Coal-meas-  "1 
ure  limestone  . .  I    Wasatch 

Waverly f  limestone . 

Nevada  limestone  J 


Ogden  quartzite. 
Ute  limestone  . . 


Cambrian 


650 
2,000 
6,000 

7,400 


1,000 
1,000 

12,000 


Clays,  marls,  and  limestones;  shal- 
low. 

Blue  aud  drab  limestones;  passing 
into  sandstones. 

Compact  sandstone  and  quartzite; 
often  reddish;  intercalations  of 
lime,  argillites,  and  conglomerate. 

Heavy  bedded  blue  and  gray  lime- 
stone, darker  near  the  base,  with 
siliceous  admixture,  especially 
near  the  top. 

Pure  quartzite,  with  conglomerate. 

Compact,  or  shaly,  siliceous  lime- 
stone. 

Siliceous  schists  and  slates,  quartz- 
ites. 


In  the  Wasatch  section  the  12,000  feet  of  metamorphosed  schists, 
slates,  and  quartzites  probably  occur  below  the  Cambrian  beds  as  exposed 
at  Eureka,  except  so  far  as  they  may  be  represented  in  the  upper  members 
by  the  Prospect  Mountain  quartzite,  while  the  great  thickness  of  Cambrian 
limestones  and  shales  of  the  Eureka  section  is  included  within  the  1,000 
feet  of  Ute  limestone  in  the  former  section.  Again,  at  Eureka  the  Permian 
at  the  top  of  the  section  is  wholly  wanting  and  the  Upper  Coal-measures, 
which  in  other  parts  of  Nevada  attain  a  development  of  nearly  2,000  feet, 


KANAB  SECTION. 


207 


the  thickness  which  has  generally  been  assigned  to  them  in  the  Wasatch, 
are  limited  to  500  feet.  It  will  be  seen,  therefore,  that  the  upper  and  lower 
portions  of  the  section  as  exposed  in  the  Wasatch,  on  the  edge  of  the 
Great  Basin,  are  wanting  in  the  Eureka  section.  Taking  out  the  12,000 
feet  of  Cambrian  at  the  base  and  2,000  feet  of  Permian  and  Upper  Coal- 
measures  from  the  summit  of  the  Wasatch  sections,  there  remains  16,000 
feet  of  strata,  which,  from  the  base  of  the  Prospect  Mountain  limestone  to 
the  top  of  the  series,  are  represented  in  the  Eureka  section  by  the  enor- 
mous development  of  28,500  feet  of  sediments. 

Mr.  C.  D.  Walcott1  constructed  a  section  across  the  entire  series  of 
Paleozoic  rocks  as  exposed  in  the  Kanab  Valley  of  the  Lower  Colorado 
in  the  plateau  province.  This  section  presents  5,000  feet  of  beds  from  the 
Cambrian  to  the  Permian  inclusive,  and  is  republished  here  as  it  offers  so 
much  that  is  of  interest  in  a  study  of  the  Paleozoic  rocks  of  the  Cordillera. 

Kanab  section,  Arizdha:  5,000  feet. 


PERMIAN,  855  feet  i 

710 

145 

835 
1,  455 

970 

100 

235 
550 

Gypaiferous  and  arenaceous  shales 
and  marls  with  impure  shaly  lime- 
stone at  base. 

I 

CARBONIFEROUS,  3,260  feet..  I 

• 
DEVONIAN,  100  feet  

Same  as  above,  with  more  massive 
limestone. 

Upper  Aubrey  

Massive  cherty  limestone,  with  gyp- 
siferous  arenaceous    bed,   passing 
down  into  calciferous  sandrock. 
Friable,  reddish  sandstone,  passing 
down  into  more  massive  and  com- 
pact sandstone  below.    A  few  fil- 
lets of  impure  limestone  interca- 
lated. 
Arenaceous   and   cherty    limestone, 
235  feet,   with   massive  limestone 
beneath.    Cherty  layers  coincident 
with  bedding  near  base. 

Red  Wall  limestone  

Sandstone  and  impure  limestone. 

CAMBRIAN,  785  feet  .  .           .  .  ) 

Massive   mottled  limestone,  with  50 
feet  sandstone  at  base. 
Thin-beddi'd,  mottled  limestone   in 
massive  layers.     Green,  arenaceous 
ami  micaceous  shales,  100  feet   at 
base. 

1 

NOTE. — Planes  of  unconformity  by  erosion  denoted  by  double  dividing  lines. 
:  Ain.  Jour.  Sci..  Sept.,  1880. 


208  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

Paleozoic  Rocks  in  British  America.— In  this  connection  attention  should  be 
called  to  the  remarkable  sections  across  the  Paleozoic  rocks  of  British 
America  exposed  along  the  line  of  the  Canadian  Pacific  Railway  where  it 
crosses  the  grandest  parts  of  the  mountains.  A  description  of  the  geolog- 
ical structure  of  the  country,  accompanied  by  maps  and  diagrams,  will  be 
found  in  a  paper  of  Mr.  R.  Gr.  McConnell,  published  in  the  reports  of  the 
Geological  Survey  of  Canada.1  The  region  described  embraces  a  belt  of 
country  about  70  miles  in  width,  and  for  the  most  part  lies  just  north  of 
the  fiftieth  parallel  of  north  latitude.  Within  this  belt  several  transverse 
sections  have  been  run  across  the  Bow  River  Valley  so  as  to  include  the 
mountains  on  both  the  east  and  west  sides.  Sections  constructed  across 
Mt.  Stephen,  Cathedral  Mountain,  and  the  Castle  Mountain  range  present 
an  instructive  sequence  of  strata  for  the  Cambrian  rocks,  while  those  in  the 
vicinity  of  Cascade  Mountain  and  along  the  Devil's  Lake  Valley  offer 
equally  good  exposures  for  tbe  Silurian,  Devonian,  and  Carboniferous. 
The  upper  members  of  the  Cambrian  are  exposed  in  both  series  of  strata, 
serving  to  connect  the  lower  with  the  upper  Paleozoic  rocks.  From  the 
standpoint  of  this  work  the  chief  value  of  the  Canadian  section  consists  in 
its  close  agreement  in  many  of  its  details  with  the  sequence  of  strata  found 
at  Eureka.  According  to  Mr.  McConnell2  the  thickness  of  the  Paleozoic 
rocks  in  the  region  explored  by  him  measures  29,000  feet.  This,  it  will  be 
seen,  is  not  far  out  from  the  thickness  given  for  the  corresponding  rocks  at 
Eureka,  where  the  best  estimates  place  the  thickness  at  30,000  feet.  The 
fossiliferous  Cambrian  limestone,  together  with  the  underlying  quartzite, 
may  be  correlated  with  the  Prospect  Mountain  quartzite  and  limestone. 
Beds  carrying  the  Olenellus  fauna  have  been  identified  in  the  Canadian 
rocks,  although  there  they  occur  far  below  the  limestone,  the  under- 
lying quartzite  having  a  much  greater  thickness  than  is  exposed  at  Eureka. 
It  is  difficult  to  determine  how  great  a  thickness  should  be  assigned  to  the 
Pogonip,  although  it  is  evidently  well  represented.  Limestones  carrying 
Halysites  are  in  many  ways  similar  to  the  Lone  Mountain  beds,  and  have  a 

1  Report  on  the  Geological  Features  of  a  portion  of  the  Rocky  Mountains.  Accompanied  by  a 
section  measured  near  the  fifty-first  parallel.  Geological  Survey  of  Canada.  Annual  Report.  New 
series,  vol.  2,  1886,  pp.  24-30. 

-  Op.  cit.,  p.  15. 


STRUCTUBAL  FEATURES.  209 

thickness  of  1,300  feet  as  against  1,800  feet  assigned  to  them  in  Nevada. 
In  the  Canadian  section  the  Devonian  exposes  only  1,500  feet  of  strata  as 
against  5,000  feet  of  Nevada  limestone,  but  on  the  other  hand  the  Carbon- 
iferous limestone  immediately  overlying  the  Devonian  exhibits  a  much 
greater  development  than  the  corresponding  horizon  at  Eureka. 

The  sequence  of  strata  in  the  Canadian  localities  shows  a  closer  agree- 
ment with  the  conditions  of  sedimentation  at  Eureka  than  do  many  expo- 
sures of  Paleozoic  rocks  situated  but  a  comparatively  short  distance  east- 
ward of  the  latter  area.  In  some  respects  the  Canadian  section  more  closely 
resembles  the  Wasatch  than  it  does  the  Eureka,  as  is  shown  in  the  great 
thickness  of  Cambrian  rocks  below  the  OUnellm  horizon.  On  the  other 
hand,  there  is  no  such  development  of  Silurian  and  Devonian  rocks  in  the 
Wasatch  as  is  shown  both  at  Eureka  and  in  Canada.  Changes  in  sedi- 
mentation appear  much  more  sudden  and  varied  in  passing  eastward  from 
Eureka  than  when  followed  northward.  In  structural  and  orographic 
features  the  two  regions  present  much  in  common,  great  lateral  com- 
pression, with  anticlinal  and  synclinal  folds,  accompanied  by  north  and 
south  lines  of  profound  displacement. 

STRUCTURAL    FEATURES. 

For  a  clear  understanding  of  the  relation  of  the  different  orographic 
blocks  to  each  other  and  to  the  numerous  outbursts  of  igneous  rocks,  a 
number  of  cross  sections  have  been  constructed  across  the  central  part  of  the 
Eureka  District.  In  one  very  marked  way  these  transverse  sections  across 
the  mountains  are  of  more  than  ordinary  interest,  as  they  bring  out  the  geo- 
logical structure  connecting  a  number  of  distinct  and,  at  the  same  time, 
interdependent  mountain  masses,  whereas  in  most  instances  in  the  Great 
Basin  sections  are  drawn  across  single  uplifted  ridges,  isolated  by  broad 
valleys  whose  recent  deposits  conceal  everything  beneath  them.  As  these 
valleys  are  frequently  from  5  to  10  miles  in  width  without  rock  exposures, 
it  is  largely  a  matter  of  conjecture  to  say  what  the  geological  structure  is 
which  underlies  them.  The  most  impressive  orographic  feature  at  Eureka 
is  the  close  relationship  between  the  anticlinal  and  synclinal  folds  to  the 
north  and  south  faults.  It  is  these  great  meridional  faults,  at  points  attain- 
MON  xx 14 


210  GEOLOGY  OF  THE  EUKEKA  DISTRICT. 

ing  a  displacement  of  13,000  feet,  that  have  determined  the  orographic 
blocks.  A  study  of  the  structural  details  as  presented  in  this  work  shows 
that  the  folding  and  flexing  of  the  beds  are  largely  due  to  lateral  com- 
pression. The  grandest  effects  of  this  lateral  compression  are  seen  along 
the  central  portions  of  the  mountains  between  the  Spring  Valley  and 
Rescue  faults.  It  is  here  that  the  greatest  energy  has  been  displayed. 
Within  these  lines  lie  the  abrupt  anticlinal  fold  of  Prospect  Ridge,  the 
sharp  synclinal  fold  of  Spring  Hill  between  the  Hoosac  and  the  Pinto,  and 
the  broad  syncline  in  the  Nevada  limestone  between  the  Pinto  and  the 
Rescue  faults.  Westward  of  Spring  Valley  the  structure  stands  out  in 
marked  contrast  with  the  mountain  blocks  included  between  these  great 
faults.  Receding  from  the  fault,  the  plication  of  strata  becomes  more  and 
more  gentle  to  the  west  without  any  violent  orographic  disturbance,  the 
limestones  falling  away  in  broad  sweeping  rolls  with  relatively  low  angles 
of  dip.  East  of  the  Rescue  fault  a  powerful  compression  of  strata  is  shown 
by  anticlinal  and  synclinal  folds  in  the  region  of  Diamond  Peak. 

Faulted  anticlines. —The  structure  whicli,  in  the  opinion  of  the  writer,  is 
most  common  in  the  Great  Basin  ranges,  that  of  a  faulted  anticline  with  a 
downthrow  along  the  axial  plane,  is  not  brought  out  in  the  sheet  of  geolog- 
ical sections,  but  is  nevertheless  well  represented  in  the  district,  both  in  the 
Fish  Creek  Mountains  and  at  Newark  Mountain.  In  both  these  uplifts 
the  displacement  is  accompanied  by  an  escarpment  along  the  fault  which  is 
coincident  with  the  axial  plane.  In  the  Fish  Creek  Mountains  we  have  a 
broad,  gently  inclined,  westerly  dipping  limestone  body,  with  the  axis  of 
the  anticlinal  near  the  eastern  edge  of  the  uplift,  the  downthrow  measuring 
about  600  feet.  On  Newark  Mountain,  which  is  a  sharp  single  ridge,  the 
downthrow  also  occurs  along  the  eastern  crest  of  the  uplift,  the  escarpment 
measuring,  approximately,  1,000  feet.  In  both  instances  the  easterly  dip- 
ping beds  extend  down  the  mountain  slope  until  lost  beneath  valley  accu- 
mulations. On  the  west  side  of  Newark  Mountain  the  flexible  White  Pine 
shale  affords  an  excellent  example  of  plication  without  fracture.  Details  in 
regard  to  the  structural  features  of  both  the  Fish  Creek  Mountains  and 
Newark  Mountain  will  be  found  in  the  chapter  devoted  to  the  descriptive 
geology  of  those  areas. 


GEOLOGICAL  CBOSS-SECTIONS.  211 

On  Prospect  Peak  we  have  a  sharp  anticlinal  fold,  with  beds  on  both 
sides  of  the  fault  standing  at  an  angle  of  70°,  but  without  any  great  amount 
of  faulting. 

In  certain  of  the  Great  Basin  ranges  the  axial  plane  occurs  along  the 
center  of  the  ridge,  as  seen  at  Ravens  Nest  in  the  Pinon  Range.  In  others 
it  follows  along  the  edge  of  the  uplift,  an  escarpment  usually  facing  the 
valley.  Of  this  latter  structure,  Newark  Mountain  is  an  excellent  example. 
In  some  of  these  ranges  there  may  be  110  faulting  of  strata  along  the  axis  of 
the  anticline ;  in  others  it  may  be  confined  to  a  few  hundred  feet,  or  the 
faulted  block  may  have  suffered  a  downthrow  measured  by  thousands  of 
feet.  In  the  latter  instance  it  is  easily  seen  that  the  strata  along  the  down- 
throw side  may  be  lost  to  sight,  being  carried  down  below  the  present  level 
of  the  Pleistocene  deposits.  Where  this  is  the  case  only  one  side  of  the 
fold  is  exposed,  leaving  a  simple  monoclinal  ridge.  This  is  what  has 
actually  occurred  in  several  of  the  narrow  uplifts  in  the  Great  Basin. 

GEOLOGICAL    CROSS-SECTIONS. 

These  sections,  atlas  sheet  xin,  will  be  readily  comprehended  when 
examined  in  connection  with  the  geological  map,  and  the  detailed  descrip- 
tions of  each  block  of  Paleozoic  sediments  as  given  in  the  text  may  be  fol- 
lowed easily  without  much  additional  explanation.  As  the  sections  have 
been  constructed,  as  far  as  possible,  across  the  strike  of  the  beds,  and  for 
the  most  part  at  right  angles  to  the  principal  meridional  lines  of  faulting, 
they  measure  with  a  considerable  degree  of  accuracy  the  amount  of  displace- 
ment and  indicate  approximately  the  compression  of  strata.  All  sections  are 
carefully  drawn  on  a  scale  of  1,600  feet  to  the  inch,  with  a  base  line  taken 
at  6,000  feet  above  sea  level,  the  height  of  the  adjacent  valleys  on  all  sides. 
So  far  as  practicable  they  have  been  selected  to  show  the  average  thickness 
of  all  sedimentary  rocks,  from  the  base  of  the  Cambrian  to  the  summit  of 
the  Carboniferous.  Of  course  all  underground  structure  is  based  upon 
observed  dips  and  strikes  taken  at  the  surface,  but  these  have  been 
obtained,  so  far  as  possible,  at  frequent  intervals  and  with  every  precaution 
which  could  be  exercised.  In  a  country  so  broken  by  faults,  dislocated  by 
igneous  outbursts,  and  where  bedding  planes  are  so  frequently  wanting,  the 


212  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

construction  of  sections  must  necessarily,  to  a  great  extent,  be  based  upon 
theoretical  reasoning. 

On  the  geological  maps,  lines  of  cross-sections  are  laid  down  by  nar- 
row black  lines  designated  at  the  borders  of  the  sheet  by  block  letters. 

Section  A-B.— This  section  is  drawn  only  halfway  across  the  Eureka 
Mountains,  and  is  confined  to  the  northeast  comer  of  the  District,  atlas 
sheet  vin.  At  the  extreme  western -end  of  the  map  the  section  exhibits  200 
or  300  feet  of  the  Hamburg  limestone  just  west  of  the  Jackson  fault,  fol- 
lowed on  the  east  side  of  the  fault  by  the  Pogonip  limestone,  which  in  turn 
is  capped  by  Eureka  quartzite  shown  on  the  summit  of  Caribou  Hill.  On 
the  east  slope  of  Caribou  Hill  the  rhyolitic  ashes  and  tuffs  conceal  the 
quartzite  and  stretching  eastward  across  the  valley  rest  against  the  steep 
wall  of  Richmond  Mountain.  These  ashes  and  tuffs  underlie  the  town  of 
Eureka,  although  nowhere  of  any  great  thickness,  and  probably  over  much 
of  this  area  overlie  solid  rhyolite  not  far  below  the  surface.  In  the  section 
the  Eureka  quartzite  is  represented  as  underlying  the  valley  as  far  eastward 
as  the  Hoosac  fault  or  the  prolongation  of  the  fault  as  recognized  south  of 
the  Richmond  Smelting  Works.  The  representation  of  a  narrow  strip  of 
Lower  Coal-measure  limestone  is  a  theoretical  deduction  based  upon  strong 
evidences  observed  at  the  latter  locality.  The  basic  andesites  of  Richmond 
Mountain  stretch  eastward  for  15,000  feet,  followed  by  7,000  feet  of  basalts, 
completely  cutting  off  all  evidences  of  any  continuation  of  either  the  Spring 
Mountain  or  County  Peak  blocks.  The  depression  of  Hunter's  Creek  and 
the  gentle  inclination  of  the  basalt  table  toward  it  are  well  brought  out  in 
cross-section.  Nowhere  are  two  epochs  of  Carboniferous  limestone  with 
the  intervening  Weber  conglomerate  better  shown  than  along  the  line  of  the 
section.  Rising  above  the  basalt  the  Upper  Coal-measures  exhibit  500  feet 
of  beds  dipping  45°  to  the  west,  resting  upon  Weber  conglomerate.  The 
conglomerate  is  inclined  at  the  same  angle,  but  soon  develops  into  an  anti- 
cline standing  at  50°  east,  followed  by  a  syncline  varying  from  50°  east  to 
30°  west.  It  is  an  admirable  example  of  the  effects  of  lateral  compression. 
The  thickness  obtained  for  the  complete  series  of  conglomerates  measures 
2,000  feet.  Underlying  the  conglomerates  come  the  Lower  Coal-measures, 
the  section  crossing  Alpha  Ridge  just  south  of  Fusilina  Peak  and  showing 


GEOLOGICAL  CROSS-SECTIONS.  213 

a  uniform  dip  to  the  west  at  an  angle  of  25°  or  30°  with  the  horizon.  With 
the  observed  dips  and  strikes  they  measure  3,700  feet.  As  described  in  the 
chapter  devoted  to  the  descriptive  geology  of  the  Diamond  Range  the 
Lower  Coal-measures  rest  unconformably  upon  the  White  Pine  shales,  the 
dip  of  the  beds  in  the  latter  epoch  reaching  an  angle  of  50°  along  the 
Newark  fault.  These  latter  shales  are  shown  to  lie  conformably  on  the 
Nevada  limestone  along  the  line  of  Hayes  Canyon.  The  anticlinal  struct- 
ure of  Newark  Mountain  is  not  brought  out  in  section,  as  the  axis  of  the 
fold  only  comes  in  near  where  the  border  of  the  map  cuts  off  the  easterly 
dipping  strata.  The  latter  stretch  far  out  into  Newark  Valley. 

Section  CD-EF.— This  section  is  constructed  across  the  central  portion  of 
the  Eureka  Mountains  (atlas  sheets  vn  and  vin),  and  stretches  in  an  east 
and  west  line  from  Antelope  to  Newark  valleys.  It  presents  more  of  the 
salient  structural  features  of  the  region  than  is  shown  in  either  of  the  other 
sections,  as  it  passes  through  the  broadest  part  of  the  Mahogany  Hills,  the 
flat-topped  summit  of  Spanish  Mountain,  the  steep  anticline  of  Prospect 
Peak,  the  syncline  of  Spring  Hill,  and  the  easterly  dipping  strata  of 
County  Peak,  and  crosses  the  Spring  Valley,  Hoosac,  and  Pinto  faults. 
The  section  starts  in  at  the  western  end  of  the  Mahogany  Hills  and  runs 
obliquely  across  the  strike  of  the  beds,  which  lie  nearly  horizontal  or 
inclined  at  very  low  angles.  In  order  to  bring  out  the  structural  features 
of  the  country,  the  Lone  Mountain  beds  are  represented  as  underlying  the 
Nevada  Devonian  above  the  base  line  of  the  section.  At  Dry  Valley  there 
is  a  considerable  but  unknown  thickness  of  valley  accumulations  with  the 
Nevada  limestones  on  the  west  side  and  the  Lone  Mountain  beds  resting 
against  the  flanks  of  Spanish  Mountain  on  the  east.  These  Lone  Mountain 
beds  lie  against  the  Eureka  quartzites  which  come  to  the  surface  on  the 
slopes  of  Spanish  Mountain  long  before  reaching  the  summit.  The  moun- 
tain presents  in  general  a  broad  anticlinal  fold,  although  broken  by  m;mer- 
ous  cross-faults  and  dislocations.  One  of  these  dislocations  is  represented 
in  the  transverse  section,  giving  a  small  exposure  of  Lone  Mountain  beds 
overlying  the  quartzite.  On  the  east  slope  of  Spanish  Mountain,  where  the 
beds  dip  away  steeply  to  the  east,  there  is  a  small  exposure  of  highly 
siliceous  limestones  without  bedding  partially  concealed  by  Quaternary 


214  GEOLOGY  OF  THE  EUREKA  DISTEICT. 

accumulations.  These,  from  their  position  resting  directly  upon  the  quartz- 
ites,  have  been  referred  to  the  Lone  Mountain  beds.  The  Spring  Valley 
fault  brings  these  Silurian  beds  up  against  the  Cambrian  of  Prospect  Ridge. 
The  section  intersects  Prospect  Ridge  just  to  the  north  of  the  summit  of 
Prospect  Peak  and  brings  out  the  anticlinal  structure  in  the  quartzite  on 
the  west  slope  overlain  on  both  sides  of  the  fold  by  the  Prospect  Mountain 
limestone  which  on  the  west  side  comes  down  to  the  line  of  the  Spring  Val- 
ley fault.  Prospect  Mountain  limestone  here  forms  the  summit  of  the 
main  ridge.  This  is  in  turn  overlain  by  Secret  Canyon  shale,  Hamburg 
limestone,  and  Hamburg  shale,  the  remaining  subdivisions  of  the  Cambrian, 
all  of  which  stand  inclined  at  about  70°  to  the  east.  As  the  section  is 
drawn  across  quite  a  high  saddle  at  the  head  of  New  York  Canyon,  con- 
necting Prospect  Peak  with  Hamburg  Ridge,  the  erosion  of  the  Secret 
Canyon  shale,  which  is  so  marked  a  feature  of  the  region,  is  not  so  well 
shown  as  it  would  be  if  the  section  were  drawn  either  to  the  north  or  south 
of  this  point,  but  it  is  sufficient  to  bring  out  the  prominence  of  the  Ham- 
burg Ridge,  which  is  everywhere  parallel  to  the  main  ridge.  Overlying 
the  Hamburg  shale  occurs  the  Pogonip  limestone  and  the  Eureka  quartzite, 
the  latter  occupying  the  slope  down  to  the  Hoosac  fault. 

At  the  base  of  the  long  uniform  slope  of  Pogonip  limestone,  which  is 
well  shown  in  the  surface  outline,  the  line  of  the  section  has  been  moved 
northward  700  feet,  in  order  to  illustrate  to  better  advantage  several  struc- 
tural features.  By  thus  moving  this  line  the  Eureka  quartzite  is  shown  on 
both  sides  of  the  homblende-audesite  body,  which  in  breaking  out  along 
the  Hoosac  fault  has  shattered  the  quartzite  all  along  the  fault.  Immedi- 
ately along  the  line  of  the  section  only  a  small  outcrop  of  the  quartzite 
occurs  to  the  east  of  the  andesite,  but  by  reference  to  the  geological  map  it 
will  be  readily  seen  that  the  exposure  forms  a  part  of  a  continuous  body  of 
considerable  extent.  East  of  the  fault  the  Carboniferous  limestones  come 
up  dipping  easterly,  but  separated  from  the  main  body  by  a  minor  fault, 
which,  so  far  as  can  be  determined,  is  accompanied  by  only  a  slight  dis- 
placement. This  is  followed  by  a  synclinal  fold,  described  elsewhere, 
lying  between  Spring  Hill  and  the  Pinto  fault.  Continuing  along  the 
line  of  the  section  east  of  the  Pinto  fault  the  Lone  Mountain  limestones  rise 


GEOLOGICAL  CROSS-SECTIONS.  215 

as  a  precipitous  wall,  abutting  against  Carboniferous  limestones,  followed 
by  a  great  development  of  Nevada  Devonian.  The  former  are  estimated 
at  1,700  feet  and  the  latter  at  4,500  feet  across  the  lower  and  middle  mem- 
bers of  the  Nevada  series.  At  Basalt  Peak  igneous  intrusions  spread  out  east- 
ward for  9,400  feet,  concealing  the  sedimentary  beds  and  completely  obscur- 
ing the  structural  features  produced  by  the  Rescue  fault.  Beneath  these 
basalts  the  section  is  constructed  wholly  upon  observed  data,  both  to  the  south 
and  east,  and  will  be  understood  by  reference  to  the  map.  From  here  to  the 
end  of  the  atlas  sheet  basalt  flows  or  Quaternary  accumulations  cover  every- 
thing with  the  exception  of  two  outcrops — one,  easterly-dipping  beds  of 
Weber  conglomerate,  almost  wholly  encircled  by  igneous  rocks,  and  the 
other,  a  low,  gentle  swell  of  Carboniferous  limestones,  rising  out  of  the 
valley  deposits  not  much  above  the  base  level  of  the  sections. 

section  GH-iK.— This  section  (atlas  sheets  ix  and  x)  is  drawn  across  the 
southern  end  of  the  mountains  in  a  continuous  line  from  west  to  east,  pass- 
ing through  Grays  Peak,  Gray  Fox  Peak,  Carbon  Ridge,  and  Century  Peak, 
and  crossing  at  right  angles  the  Hoosac,  Pinto,  and  Rescue  faults.  It 
crosses  the  mountains  about  4£  miles  south  of  section  CD-EF,  and  for  the 
most  part  runs  along  the  extreme  southern  end  of  the  different  mountain 
blocks,  touching,  however,  the  Fish  Creek  Mountains  at  their  northern  end, 
but  passing  far  to  the  south  of  the  Diamond  Range.  The  section  passes 
along  the  base  of  the  Mahogany  Hills,  following  the  Lone  Mountain  lime- 
stones approximately  parallel  with  their  strike,  the  beds  dipping  northward 
into  the  hills  beneath  the  Nevada  Devonian.  Across  Spring  Valley,  for  a 
width  of  6,000  feet,  all  Paleozoic  strata  are  concealed  beneath  the  Quater- 
nary, but  with  the  rising  of  the  hills  on  the  east  side  the  Pogonip  lime- 
stones come  in  capped  on  the  summit  of  the  ridge  by  the  Eureka  quartz- 
ites.  Here  the  broad  anticlinal  structure  of  the  Fish  Creek  Mountains  is 
clearly  brought  out,  but  without  the  fault  recognized  on  the  east  side  of  the 
higher  portion  of  the  mountains.  East  of  Castle  Mountain  there  occurs  a 
shallow  basin  or  depression  in  quartzite  occupied  by  Lone  Mountain  and 
Devonian  beds,  beyond  which  there  is  a  second  anticline,  with  the  Eureka 
beds  arching  over  the  summit.  At  Lamoureux  Canyon  occurs  a  slight  dis- 
placement, the  walls  on  both  sides  where  the  section  crosses  exposing  the 


216  GEOLOGY  OF  THE  EUEEKA  DISTRICT. 

quartzite  in  abrupt  escarpments.  Between  Lamoureux  Canyon  fault  and 
Pinnacle  Peak  fault  the  mountain  mass  presents  another  broad  anticlinal 
mass,  of  which  Grays  Peak  forms  the  summit.  The  beds  on  the  peak  lie 
nearly  horizontal,  falling  away  on  both  sides  at  relatively  high  angles. 
Toward  Pinnacle  Peak  the  Nevada  limestones  come  in,  resting  unconform- 
ably  against  the  uplifted  block  between  Pinnacle  Peak  and  Lookout  Moun- 
tain faults.  Between  these  two  latter  faults,  measuring  on  the  surface  but 
scarcely  more  than  2,000  feet,  the  only  beds  exposed  are  the  Eureka 
quartzites,  dipping  eastward  and  capping  Pinnacle  Peak.  East  of  the 
Lookout  Mountain  fault  there  is  a  block  of  Pogonip  limestone,  beyond 
which  comes  the  Prospect  Mountain  Ridge  uplift.  The  only  Cambrian 
rocks  found  on  the  surface  are  the  Prospect  Mountain  limestones,  the  over- 
lying horizons,  together  with  the  Pogonip  limestone  and  Eureka  quartzite 
of  the  Silurian,  being  either  buried  beneath  flows  of  rhyolite  or  Quaternary 
accumulations.  About  a  quarter  of  a  mile  to  the  north  of  the  line  of  this 
section  the  entire  Cambrian  series  is  exposed,  and  the  beds  are  introduced 
here  very  much  as  they  are  found  beyond  the  line  of  the  rhyolites  and 
pumices.  Between  the  Hoosac  and  Pinto  faults  the  section  again  crosses 
the  Carboniferous  block,  which  here  includes  nearly  all  of  the  Weber  con- 
glomerate, as  well  as  the  Lower  Coal-measure  limestones,  both  members 
lying  at  angles  inclined  from  60°  to  70°  to  the  east.  Along  the  Pinto  fault 
the  line  of  contact  is  obscured  by  tuff's  and  pumices.  Along  the  southern 
extremity  of  the  Silverado  Mountains  no  structural  evidences  were  obtained, 
as  the  underlying  rocks  are  for  the  most  part  concealed  by  tuff's  and  puma- 
ceous  material,  but  in  the  section  the  lavas  are  represented  as  overlying 
Lone  Mountain  beds,  as  the  latter  are  found  higher  up  in  the  foothills  above 
the  line  of  igneous  rocks.  At  the  entrance  of  Rescue  Canyon  the  rhyolites, 
which  break  through  the  Nevada  limestone  along  the  line  of  the  Rescue 
fault,  are  represented  with  a  width  of  nearly  3,000  feet.  The  section 
crosses  the  summit  of  Century  Peak  and  brings  out  clearly  the  anticlinal 
structure  of  the  limestone  ridge  on  the  east  side  of  Rescue  Canyon.  On 
the  east  side  of  the  peak  the  beds  gradually  fall  away  toward  Newark 
Valley  and  are  lost  beneath  the  Quaternary  deposits. 


PLATE  SECTIONS.  217 

On  PI.  ii  of  this  volume  will  be  found  a  geological  section  drawn 
across  the  summit  of  Prospect  Peak,  exhibiting  the  relations  of  the  great 
body  of  Cambrian  limestone  to  the  crest  of  the  ridge,  the  limestone  here 
rising  to  the  top  of  the  peak.  The  position  of  Prospect  Ridge  to  the  Ham- 
burg Ridge  is  also  more  clearly  shown  than  in  the  general  section.  The 
same  series  of  rocks  at  the  northern  end  of  the  ridge  across  Ruby  Hill  and 
Adams  Hill  are  also  shown  on  this  plate.  On  the  same  plate  a  section  is 
given  across  Pinto  (Peak,  and  about  1  £  miles  to  the  northward  of  the  latter 
section  is  another  east  and  west  section,  drawn  from  the  Pinto  to  the 
Hoosac  fault. 


CHAPTER  VII. 

PRE-TERTIARY  IGNEOUS  ROCKS. 

Igneous  rocks  have  played  a  most  important  part  in  the  development 
of  the  geological  history  of  the  Eureka  District.  They  may  be  separated 
into  two  distinct  groups :  first,  those  which  reached  their  present  position  in 
pre-Tertiary  times;  second,  a  younger  and  much  more  extended  series  of 
eruptions,  those  of  Tertiary  and  post-Tertiary  age.  Not  only  do  they  be- 
long to  distinct  geological  periods,  but  their  mode  of  occurrence  is  quite 
unlike  and  their  petrographical  characters  in  every  way  different. 

Granite,  granite-porphyry,  and  quartz-porphyry  are  the  types  of  the 
pre-Tertiary  rocks.  Their  surface  exposures  are  very  restricted,  being 
quite  insignificant  as  compared  with  the  more  recent  volcanic  lavas,  and 
only  to  a  very  limited  degree  have  their  extrusions  influenced  the  present 
physical  features  of  the  country. 

Granite.— Between  the  Sierra  and  the  Wasatch  there  are  probably  few  of 
the  many  longitudinal  ranges  which  rib  the  Great  Basin,  other  than  those 
made  up  entirely  of  volcanic  lavas,  that  do  not  show  one  or  more  bodies  of 
granite  or  crystalline  schists  of  greater  or  less  extent.  Along  the  lines  of 
upheaval  of  one  or  two  of  these  ranges,  the  accumulations  of  recent  lavas 
have  been  on  so  vast  a  scale  that  all  direct  evidences  of  an  older  preexist- 
ing range  are  to-day  wholly  wanting.  In  some  instances  granite  and  gneisses 
cover  large  tracts  of  country  and  occasionally  culminate  in  peaks  rising 
high  above  the  surrounding  regions,  but  so  abrupt  are  the  changes  in  the 
Archean  topography  that  they  occur  for  the  most  part  only  as  subordinate 
exposures  over  limited  areas.  The  granite  is  found  cropping  out  along  the 
base  of  the  foot-hills  beneath  the  Paleozoic  sediments,  occasionally  occupy- 
ing low  passes  through  breaks  in  the  ranges,  or,  as  is  frequently  the  case, 
they  are  associated  with  extrusions  of  volcanic  lavas  and  accidentally  left 
bare  or  else  uncovered  by  recent  erosion.  Westward  of  the  Salt  Lake  Basin, 

218 


GRAOTTE.  219 

granite  is  found  in  isolated  patches  in  such  uplifts  as  the  Ombe,  Gosiute,  and 
Peoquop  ranges.  The  East  Humboldt  Mountains  present  the  grandest  mass 
of  the  older  crystalline  rocks,  stretching  with  the  trend  of  the  range  over 
sixty  miles  in  a  nearly  north  and  south  direction,  and  is  the  most  extensive 
area  of  pre-Cambrian  rocks  to  be  found  in  central  Utah  and  Nevada  along 
the  country  examined  by  the  Fortieth  Parallel  Exploration.  Immediately 
westward  of  this  latter  range  in  the  country  occupied  by  the  Diamond  and 
Pinon  ranges,  no  exposures  of  granites  occur,  and  it  is  one  of  the 
largest  areas  known  in  the  Great  Basin,  in  which  all  evidences  of  granite 
and  of  an  Archean  body  are  wanting.  There  are  good  reasons  for  believ- 
ing that  in  early  Paleozoic  time  this  area  east  of  the  Humboldt  Mountains 
was  a  deep  trough  of  the  sea,  in  which  Cambrian,  Silurian  and  Devonian 
sediments  were  deposited.  At  all  events,  the  mountain  ranges  within  this 
region  offer  excellent  sections  of  the  lower  Paleozoic  strata,  which  over  large 
areas  of  the  Great  Basin  are  unknown. 

At  Eureka,  where  the  Diamond  and  Pinon  ranges  unite  in  a  broad  ele- 
vated mass  of  sedimentary  beds  of  great  thickness,  singularly  broken  up  by 
great  faults  into  bold  mountain  ridges,  only  one  obscure  outcrop  of  granite 
is  known.  It  is,  however,  not  without  considerable  interest,  and  when  the 
geological  position  of  these  granites  and  allied  rocks  of  the  Great  Basin 
come  to  be  studied  in  detail  in  their  relation  to  the  Archean  masses  and  the 
uplifts  of  the  parallel  ranges,  it  may  be  found  to  throw  much  light  upon 
some  complex  structural  problems.  Here  at  Eureka  the  outcrop  does  not 
occur  rising  above  the  Quaternary  plain  along  the  base  of  a  ridge,  nor  in 
the  bottom  of  some  deeply  eroded  canyon.  On  the  contrary  it  is  found 
1,000  feet  above  the  level  of  Diamond  Valley,  at  the  extreme  northern  end 
of  Prospect  Ridge,  on  the  steep  south  slope  of  the  ravine  which  separates 
Ruby  Hill  from  the  main  ridge.  It  is  so  insignificant  and  so  covered  with 
deTjris  derived  from  the  limestone  hills  above  that  it  might  easily  escape 
attention,  especially  as  it  presents  no  inequalities  of  surface.  This  granite 
is  best  seen  on  comingup  the  ravine  from  the  west,  and  is  exposed  just  above 
the  path,  some  miners  having  cut  into  it,  attracted  by  the  red  color  of  the 
decomposed  rock.  It  extends  from  the  footpath  up  the  steep  slope  for  300 
feet  to  within  50  feet  of  the  top  of  the  hill. 


220  GEOLOGY  OF  THE  ETJEEKA  DISTEICT. 

Although  upon  the  surface  this  granite  body  is  limited  in  extent,  there 
is  reason  to  suppose  that  it  represents  a  much  larger  mass  and  has  exerted  a 
considerable  influence  on  the  geological  structure  of  the  ridge.  Prospect 
Mountain  quartzite,  the  oldest  sedimentary  beds  in  the  District,  surrounds 
the  granite  upon  the  northwest  and  northeast  slopes  and  dips  away  from  it 
in  irregular  broken  masses.  To  the  south  the  Prospect  Mountain  lime- 
stones, which  form  the  main  ridge,  occur  resting  directly  upon  the  granite. 
That  this  exposure  of  granite  along  the  steep  slopes  of  the  ravine  is  due  to 
the  erosion  of  the  crushed  quartzite  can  not  well  be  questioned,  and  the 
ravine  itself  probably  owes  its  existence  to  the  fracturing  of  the  quartzite 
from  some  upward  orographic  movement  of  the  granite.  The  farther  away 
the  quartzite  lies  from  the  granitic  center,  the  less  disturbance  is  seen  in 
the  beds. 

The  age  of  the  granite  is  unknown.  Quite  possibly  it  formed  a 
portion  of  an  Archean  body  around  which  the  sands  were  deposited, 
subsequent  orographic  movements  disturbing  and  breaking  tip  the  sedi- 
mentary beds.  If  the  granite  is  later  than  the  sedimentary  beds  it  only 
broke  through  the  quartzite  at  the  northern  end  of  the  range  and  failed  to 
cut  the  limestone.  No  intrusive  dikes  of  granite  are  known  penetrating 
the  overlying  beds.  At  the  time  of  the  sinking  of  the  Richmond  shaft  the 
workmen  found,  at  a  depth  of  1,200  feet  from  the  surface,  near  the  bottom 
of  the  shaft,  fragments  of  a  decomposed  crystalline  rock.  They  were  so 
highly  altered  that  the  microscope  failed  to  detect  with  certainty  the  nature 
of  the  rock,  though  it  carried  quartz  and  mica  and  kaolinized  feldspar. 
This  evidence,  though  slight,  would  indicate  an  off-shore  deposit  for  the 
quartzites. 

A  peculiarity  of  this  granite  is  the  varied  texture  it  offers  over  so 
small  an  area,  presenting,  however,  a  true  granitic  habit.  The  essential 
minerals  are  quartz,  orthoclase,  plagioclase,  hornblende,  and  mica,  the  lat- 
ter being  very  abundant.  The  quartz  is  grayish  white  in  color,  in  irregular, 
pellucid  grains.  Hornblende  is  apparently  more  abundant  in  the  fine  than 
in  the  coarse  grained  varieties. 

Quartz-porphyry.— Only  two  small  exposures  of  quartz-porphyry  have  been 
observed,  both  occurring  on  Mineral  Point,  north  of  Adams  Hill,  and, 


GRANITE-PORPHYRY.  221 

although  separated  by  limestone,  it  seems  probable  that  below  the  surface 
the  two  bodies  have  a  common  origin,  inasmuch  as  on  top  they  present 
nearly  identical  features.  Neither  body  rises  above  the  general  level  of  the 
country,  erosion  wearing  away  equally  both  limestone  and  dike. 

The  larger  outburst  is  an  irregular  body  with  an  east  and  west  trend; 
the  other,  a  few  hundred  yards  northward,  and  a  short  distance  beyond 
the  Bullwhacker  mine,  occurs  as  a  dike  from  300  to  400  feet  long,  with  a 
direction  N.  20°  W.  Both  quartz-porphyry  exposures  are  much  decom- 
posed and  discolored  by  oxide  of  iron.  Under  the  microscope  they  show, 
however,  orthoclase,  quartz  and  mica,  with  characteristic  isotropic  glass. 
The  feldspars  are  altered  into  kaolin,  with  a  secondary  formation  of  potash 
mica;  the  quartz  grains  carry  both  liquid  and  glass  inclusions.  In  the 
Bullwhacker  mine  the  porphyry  is  seen  as  a  dike,  striking  north  18°  W., 
with  a  dip  35°  to  the  east.  In  one  place,  at  least,  on  the  main  level  it 
occurs  as  a  white  rock,  differing  from  the  surface  exposure  not  only  in 
color,  but  by  a  relatively  large  amount  of  quartz  grains  and  by  a  develop- 
ment of  numerous  modified  cubes  of  pyrites.  The  pyrites  in  the  dike,  in 
distinction  from  that  carried  by  the  surface  body,  is  perfectly  fresh  and 
may  account  to  some  extent  for  difference  in  color.  It  is  probable  that  the 
pyrites  is  a  secondary  product,  formed  after  the  cooling  of  the  original 
magma.  Careful  assays  show  that  the  pyrites  carries  both  gold  and  silver. 
The  quartz-porphyry  differs  structurally  from  all  other  igneous  rocks  in  the 
region,  being  distinct  from  granite  on  the  one  hand  and  from  rhyolite  on 
the  other.  None  of  the  rhyolites  at  all  resemble  it.  It  is  possible  that  it 
occurs  as  an  offshoot  from  a  large  body  of  granite,  the  structural  features 
being  due  to  conditions  of  cooling.  No  direct  evidence  of  the  age  of  the 
quartz-porphyry  is  afforded  other  than  that  it  penetrates  the  Pogonip  lime- 
stone of  the  Silurian. 

Granite-porphyry.— The  important  granite-porphyry  bodies  are  confined  to 
the  country  lying  between  the  Fish  Creek  Mountains  and  the  Mahogany 
Hills.  They  occur  in  two  large  masses  separated  by  a  narrow  belt  of  lime- 
stone with  a  few  smaller  outlying  exposures,  probably  offshoots  from  the 
parent  mass.  The  principal  exposure  of  this  porphyry  lies  immediately  to 
the  west  of  Wood  Cone  along  the  southern  base  of  the  Mahogany  Hills, 


222  GEOLOGY  OF  THE  EUEEKA  DISTRICT. 

and  with  an  undulating  surface  gradually  falls  away  to  the  broad  valley 
beyond  the  limits  of  the  map.  Its  extension  westward  is  lost  beneath  the 
Quaternary.  It  presents  an  oval-shaped  mass  one  mile  and  a  quarter  long 
by  three-quarters  of  a  mile  wide,  partially  hemmed  in  on  both  the  north 
and  south  sides  by  ridges  of  Silurian  limestones  which  rest  against  it.  For 
the  most  part  it  is  a  coarse  grained  rock  disintegrating  readily  under  atmos- 
pheric agencies.  Much  of  it  might  easily  pass  for  granite,  but  other  por- 
tions possess  a  decidedly  porphyritic  structure,  especially  along  the  line  of 
contact  with  the  Pogonip  limestone  on  the  south.  Geologically,  a  more 
important  body  is  the  bold  prominent  dike  trending  approximately  north 
and  south  for  over  2  miles,  with  a  varying  width,  measuring  across  its 
broadest  expansion  nearly  1,000  feet.  At  the  surface  no  direct  connection 
can  be  traced  between  the  main  granitic  area  and  the  dike,  the  continuity 
being  broken  by  an  arch  or  saddle  of  Pogonip  limestone  which  passes 
beneath  the  Eureka  quartzite  of  Wood  Cone.  This  limestone  saddle  is 
scarcely  more  than  300  hundred  feet  in  width.  There  can  be  no  reason- 
able doubt  that  the  granite-porphyry  dike  is  an  offshoot  from  the  main 
body  in  much  the  same  way  as  the  branch  dikes  described  farther  on  are 
related  to  the  larger  one.  A  strong-  proof  of  this  connection  is  seen  in  an 
isolated  exposure  of  the  crystalline  rock  worn  bare  by  erosion  on  the  sum- 
mit of  the  limestone  saddle. 

The  southern  end  of  the  main  dike  contracts  to  one  hundred  feet  or 
less  in  width  and  splits  up  into  numerous  small  dikes  ramifying  in  various 
directions,  becoming  finally  lost  in  the  limestone.  Between  the  surface 
outlines  of  the  two  walls  of  the  dike  there  exists  a  marked  contrast.  On 
the  one  side  the  western  wall  curves  slightly  and  regularly  in  sharply 
defined  lines.  On  the  other  the  eastern  side  presents  a  most  irregular 
outline,  caused  by  numerous  branch  dikes,  offshoots  from  the  main  dike, 
trending  in  nearly  parallel  courses  in  a  northeastern  direction.  They  break 
the  regularity  of  outline,  besides  fracturing  and  displacing  the  limestone 
beds.  Fig.  5  shows  the  position  of  the  secondary  dikes  to  the  primary 
one  and  is  a  reproduction  on  a  reduced  scale  of  a  portion  of  the  map. 
The  conventional  signs  indicate  approximately  the  strike  and  dip  of  the 
adjoining  limestone. 


GRANITE-POKPHYEY. 


223 


Of  these  branch  dikes  from  the  parent  stock  the  longest  has  been 
traced  on  the  surface  for  nearly  2  miles,  gradually  narrowing  to  a  few  feet 
and  breaking  up  into  stringers  and  veins  of  granite-porphyry  at  the  north- 
east end  in  the'  same  manner  as  the  main  dike.  Other  members  of  this  sys- 
tem of  parallel  dikes  may  be  equally 
as  persistent  in  their  trend,  without 
appearing  on  the  surface,  erosion  hav- 
ing failed  to  lay  them  bare.  They  vary 
from  25  to  250  feet  in  width,  but  all 
present  much  the  same  physical  con- 
ditions as  regards  their  occurrence  and 
relation  to  the  limestone.  On  the  same 
general  course  two  narrow  dikes  pene- 
trate the  limestone  just  west  of  Castle 
Mountain,  and  it  is  quite  possible  that 
they  belong  to  the  same  system  as  the 
others  if  not  actually  connected  with 
them  beneath  the  surface.  The  absence 
of  offshoots  on  the  west  side  of  the 

main  dike  and  their  frequency  and  per-  GmruleporpJiyry 

sistency  on  the  east  are  not  without  Fia-  s.-Granite-porphyry  dike, 

geological  interest.  By  reference  to  atlas  sheet  xi  the  position  of  the  dikes 
to  the  main  granite-porphyry  body  and  the  Fish  Creek  Mountains  and 
Mahogany  Hills  may  be  readily  seen. 

Limestone  adjoining  Porphyry.— The  main  granite-porphyry  dike,  like  the 
quartz-porphyry,  breaks  through  the  Pogonip,  and  the  smaller  dikes  on 
Castle  Mountain  break  through  the  Lone  Mountain,  but  all  other  evidence 
as  to  their  age  is  wanting.  Their  mode  of  occurrence  and  their  relations 
to  the  orographic  blocks  are  quite  unlike  the  extrusions  of  the  Tertiary 
lavas.  Although  both  sides  of  the  main  dike  lie  in  limestone  of  the  Pogo- 
nip epoch,  the  beds  have  undergone  considerable  displacement;  those  on 
the  east  side  belonging  to  a  higher  horizon  than  those  found  in  contact  with 
the  dike  at  the  north  end  on  the  west  side.  Immediately  adjoining  the 


224  GEOLOGY  OF  THE  EUEEKA  DISTEICT. 

dike  at  the  north  end  the  limestone  stands  at  angles  between  70°  and 
90°,  and  farther  south  only  35°  to  45°,  but  for  the  greater  part  of  the  dis- 
tance along  the  contact  crumpling  and  metamorphism  have  so  altered  it  as 
to  have  obliterated  all  signs  of  stratification.  Owing  to  the  frequency  of 
the  secondary  dikes  this  metamorphism  of  strata  is  much  more  noticeable  on 
the  east  than  on  the  west  side.  Instances  may  be  seen  where  the  lime- 
stone is  completely  altered  into  a  fine  crystalline  white  marble.  Nowhere, 
however,  does  the  alteration  of  strata  produced  by  heat  penetrate  the  lime- 
stone for  any  great  distance  from  the  dikes,  not  even  between  those  which 
run  parallel  to  each  other  only  a  few  hundred  yards  apart.  The  effects  of 
heat  are  shown  far  more  on  the  cooling  and  crystallization  of  the  intrusive 
molten  mass  than  on  the  cold  contact  rocks.  As  the  beds  recede  from  the 
main  dike  the  dip  becomes  less  and  less  steep,  the  stratification  less 
obscure,  and  toward  the  southern  end  of  the  dike  the  limestones  lie  nearly 
horizontal. 

The  main  porphyry  dike  in  cutting  the  limestone  follows  closely 
the  strike  of  the  beds,  whereas  the  branch  dikes  trending  approximately  at 
right  angles  to  the  main  one  run  across  the  strata.  The  position  of  the 
main  dike  is  determined  in  part  by  a  line  of  faulting  and  in  part  by  an  anti- 
clinal fold.  South  of  Wood  Cone,  where  the  dike  first  makes  its  app'ear- 
ance,  it  trends  off  to  the  southwest  along  the  fault  line,  maintaining  this 
course  until  near  the  summit  of  the  limestone  hill  which  lies  midway 
between  the  two  wagon  roads  that  cross  the  dike.  Here  meeting  the  anti- 
cline, it  curves  slightly  and  follows  the  axis  of  the  fold  to  the  southeast. 
This  anticline  may  be  traced  southward  beyond  the  surface  outcrop  of  the 
granite-porphyry,  as  is  indicated  by  tlie  dips  and  strikes  on  the  map. 
Along  the  northern  end  of  the  dike  the  amount  of  displacement  can  not 
be  determined  beyond  the  fact  already  stated,  that  both  sides  of  the  fault 
lie  in  the  Pogonip.  Why  the  secondary  dikes  should  break  out  approxi- 
mately at  right  angles  to  the  main  one  and  not  parallel  with  it,  it  is  diffi- 
cult to  say.  It  would  seem  as  if  lines  of  least  resistance  would  have  been 
formed  parallel  with  the  line  of  the  axial  fold  and  the  line  of  displace- 
ment along  which  the  main  dike  reached  the  surface.  On  the  east  side  the 
beds  are  a  pure  crystalline  limestone,  uniform  in  texture  and  bluish  gray 


VARIATIONS  JN  GRANITE  PORPHYRY.  225 

in  color.  The  lower  beds  on  the  west  side  are  darker  in  color,  more  sili- 
ceous in  composition,  and  rich  in  black  cherty  nodules,  characteristics  which 
everywhere  define  the  upper  from  the  lower  beds  of  the  Pogonip  lime- 
stone. 

That  the  strata  near  the  southwest  end  of  the  dike  belong-  to  the  upper 
portion  of  the  Pogonip  epoch  is  evidenced  by  the  patches  of  Eureka 
quartzite  still  in  place  and  by  a  number  of  loose  bowlders  and  quartzite 
debris  scattered  over  the  hill  slopes. 

In  the  middle  of  the  main  dike,  just  north  of  the  road  which  follows 
the  Spring  Valley  drainage  channel,  there  occurs  a  curious  bit  of  cherty 
limestone.  It  is  several  hundred  yards  in  length,  but  only  a  few  feet  in 
width,  and  lies  completely  surrounded  by  granite-porphyry,  which  may  be 
seen  penetrating  and  filling  up  the  irregular  outline  in  the  limestone.  In 
places  the  molten  mass  appears  to  have  eaten  into  the  sedimentary  body, 
although  only  to  a  very  limited  extent.  Along  the  contact  both  the  por- 
phyry and  limestone  present  the  same  phenomena  of  cooling  as  seen  near 
the  outer  walls  of  the  main  dike.  Even  this  narrow  body  of  limestone 
does  not  appear  to  have  undergone  much  metamorphism,  except  along  the 
contact. 

Structural  Variatinos  in   Granite-porphyry.— The    chief  interest    attached    tO    the 

granite-porphyry  lies  in  the  very  variable  structural  differences  produced 
in  the  erupted  material  of  the  dike,  differences  which  are  mainly  dependent 
upon  the  chilling  effect  of  cold  contact  walls  upon  a  rapidly  cooling  molten 
mass.  The  width  of  the  dikes  has  much  to  do  in  determining  these  physical 
conditions  governing  crystallization.  In  other  words,  development  of  crys- 
tallization is  dependent  upon  rate  of  cooling,  and  in  narrow  dikes  a  molten 
magma  is  more  rapidly  chilled  than  in  broader  bodies.  There  are  probably 
few  localities  in  the  Great  Basin  where  the  results  of  rapid  chilling  and 
crystallization  of  a  granite  magma  in  narrow  dikes  along  several  miles  of 
contact  walls  can  be  studied  to  better  advantage  or  are  more  worthy  of  a 
detailed  petrographical  investigation.  For  petrographical  details  the  reader 
is  referred  to  the  paper  by  Mr.  Joseph  P.  Iddings. 

The  large  oval-sha.ped  area  to  the  north  and  the  broader  central  por- 
tions of  the  main  dike  are  quite  similar  rocks,  presenting  the  characteristics 
MON.XX 15 


226  GEOLOGY  OF  THE  EUliEKA  DISTRICT. 

of  a  coarse  grained  granite  composed  of  pellucid  quartz,  dark  brown  biotite, 
and  orthoclase  in  Carlsbad  twins,  with  varying  proportions  of  plagioclase 
and  strongly  pleochroic  hornblende.  It  everywhere  weathers  in  rounded 
masses  with  rough  surfaces,  disintegrating  like  many  varieties  of  granite. 
It  is,  however,  only  a  limited  portion  of  the  dike  which  possesses  anything 
like  a  granitic  structure,  the  greater  part  of  the  main  dike  and  all  the 
secondary  branches  having  a  decidedly  porphyritic  structure.  All  through 
the  central  part  of  the  dike  the  rock  is  formed  of  large  ill-defined  crystals, 
porphyritically  imbedded  in  a  groundmass  of  the  same  composition,  which 
under  the  microscope  is  seen  to  possess  a  micro-granitic  structure.  This 
porphyritic  structure  can  be  traced  for  miles  along  all  the  dikes  parallel  with 
their  course.  From  the  normal  type  in  the  central  portion  of  the  dike 
toward  the  outer  limestone  wall  there  is  a  gradual  and  at  first  an  almost 
imperceptible  transition  to  the  finer  grained  rock,  with  more  and  more  of 
the  porphyritic  and  less  of  the  granitic  structure.  Across  these  transitional 
rocks  the  mineral  components  remain  the  same,  the  differences  consisting  in 
the  size  of  the  grains  and  their  relative  proportions  and  structural  relations. 

All  thin  sections  show  the  rock  to  be  entirely  crystalline  without 
isotropic  glass.  At  a  distance  of  from  20  to  30  feet  from  the  limestone,  vary- 
ing with  the  width  and  position  of  the  dike,the  rock  shows  a  marked  por- 
phyritic habit,  though  the  larger  crystals  are  still  in  excess.  In  the  nar- 
rower dikes  the  transitions  are  not  so  well  shown  as  the  coarser  portions  and 
are  less  characteristically  developed  and  the  changes  far  less  gradual.  In 
the  latter  the  quartz  is  very  abundant  and  frequently  occurs  in  well  devel- 
oped dihexahedrons,  in  strong  contrast  to  the  quartz,  with  irregular  outlines, 
as  seen  in  the  granitic  structure.  From  here  to  the  limestone  contact  the 
change  is  more  rapid,  the  larger  crystals  becoming  less  and  less  abundant, 
being  replaced  by  more  and  more  micro-crystalline  goundmass. 

Nearly  everywhere  along  the  immediate  line  of  contact  the  rock  pre- 
sents the  habit  of  a  quartz-porphyry  made  up  of  a  crystalline  groundmass, 
with  well  developed  crystals  of  quartz  and  orthoclase,  and  still  accompanied 
by  some  plagioclase.  Both  mica  and  hornblende  usually  fall  off  in  amount 
toward  the  edge  of  the  dike,  in  the  more  porphyritic  rocks,  the  hornblende 
being  present  only  in  the  granitic  types,  and  the  first  to  disappear  with 


VARIATIONS  IN  GRANITE-PORPHYRY.  227 

structural  changes.  With  this  change  in  structure  the  rock  becomes  more 
compact  and  weathers  in  angular  blocks  with  smooth  surfaces,  the  contact 
products  offering  the  greatest  possible  contrast  with  the  central  portion  of 
the  dike,  which  weathers  in  rounded  masses  with  rough  surfaces,  disintegra- 
ting easily  under  atmospheric  influences.  Yet  in  the  wider  dikes  these 
changes  can  be  traced  so  readily,  step  by  step,  from  the  one  rock  into  the 
other,  that  the  evidence  is  clear  that  they  are  but  different  structural 
developments  of  the  same  erupted  material,  but  not  necessarily  identical  in 
chemical  composition  at  the  time  crystallization  took  place.  In  crossing 
the  dikes  one  passes  within  100  yards,  over  excellent  quartz-porphyry,  on  to 
normal  granite-porphyry,  and  then  on  to  a  rock  which  can  not  be  told  from 
many  varieties  of  granite ;  so  that  one  is  forced  to  believe  that  the  only  differ- 
ences between  granite  and  granite-porphyry  is  in  many  cases  purely  one  of 
structure,  dependent  upon  conditions  in  cooling  rather  than  upon  any 
differences  in  age  or  chemical  constitution  of  the  original  magma. 

A  study  of  the  dikes  makes  it  evident  that  there  could  not  have  been 
any  forcing  of  lava  into  the  middle  of  a  dike  already  partially  occupied 
by  an  earlier  crystalline  rock.  As  the  branch  dikes  are  mostly  narrow,  the 
granitic  and  normal  granite-porphyry  structures  are  less  fully  developed 
and  are  frequently  wanting,  the  effects  of  chilling  and  rapid  cooling  from 
both  walls  toward  the  interior  producing  only  the  types  of  quartz-porphyry 
developed  along  the  walls  of  the  broader  dike.  Another  striking  feature  of 
the  rapid  cooling  of  the  magma  is  seen  in  the  marked  tendency  of  the 
crystalline  rock  to  develop  a  jointed  structure  near  the  lines  of  contact,  in 
planes  parallel  to  the  walls. 

In  places  the  porphyry  contacts  present  a  fissile,  sherdy  structure, 
lines  of  parting  becoming  wider  and  wider  apart  toward  the  center  of  the 
dikes  and  gradually  disappearing.  In  the  jointed  portion  the  rock  is  always 
fine  grained,  and  frequently  possesses  an  aphanitic  structure,  the  mineral 
components,  however,  remaining  the  same.  The  rock  frequently  undergoes 
marked  changes  in  color  in  passing  from  the  coarse  grained  granitic  structure 
to  the  contact  rock.  In  these  changes  it  will  frequently  pass  from  light 
gray  into  dark  gray,  blue,  and  along  the  contact  becoming  almost  black. 


228 


GEOLOGY  OF  THE  EUREKA  DISTRICT. 


Chemical  Composition  of  Granite-porphyry.— The    original   magma    injected    into 

the  limestone  through  the  various  openings  was,  so  far  as  can  be  told,  much 
the  same  in  its  ultimate  composition.  It  is  probable  that  the  variation  in 
chemical  composition  between  rocks  of  different  dikes  is  no  greater  than 
that  between  different  parts  of  the  same  dike.  While  structural  peculiar- 
ities across  the  dike  are  strongly  marked,  the  mineral  constituents  remain 
much  the  same,  although  the  walls  are  more  acid  than  the  center  and  carry 
less  ferro-magnesian  minerals.  As  regards  tenure  of  silica  the  variation 
between  one  rock  and  another  would  not  exceed  5  per  cent,  with  corre- 
sponding variations  in  their  essential  ingredients.  It  is  probable  that  the 
silica  variations  in  the  bulk  of  the  granite-porphyry  would  fall  within  4 
per  cent. 

The  following  two  analyses  of  granite-porphyry  were  made  by  Mr. 
Andrew  A.  Blair: 


No.  1. 

No.  2. 

Silica       

Percent. 
68-68 

Percent. 
72-01 

Alumina.  .  .. 

16-28 

15-51 

Ferric  oxide.  .. 

•66 

Ferrous  oxide    

2-55 

1-36 

Lime  

2-24 

1-35 

Magnesia  

•81 

•51 

Soda    ...             

2-88 

2-36 

Potash  

4-07 

4-71 

Titanic  acid  

•05 

Not  det. 

•17 

•33 

Water  

•68 

1-24 

Total  

99-07 

99-38 

Analysis  No.  1  may  be  taken  as  representing  the  composition  of  a 
large  area  of  the  rock  west  of  Wood  Cone  possessing  a  granitic  structure, 
the  essential  minerals  having  no  crystallographic  outline.  The  quartz  and 
feldspar  are  of  medium  size  and  are  accompanied  by  nearly  all  the  accessory 
minerals  recognized  in  these  rocks,  including  biotite,  zircon,  titanite,  and 
allanite. 

Analysis  No.  2  is  made  from  granite-porphyry  obtained  from  the  mid- 


COMPOSITION  OF  GRANITE-PORPHYEY.  229 

die  of  a  dike  about  30  feet  in  width.  It  is  probably  slightly  more  acidic 
than  the  mass  of  the  rock.  The  groundmass  of  the  rock  is  made  up  of  an 
aggregation  of  quartz  and  feldspar,  the  former  crystallized  in  regular  dihexa- 
hedrons.  Biotite  is  present,  but  no  hornblende,  and  in  the  hand  specimens 
there  is  only  a  slight  development  of  ferro-magnesian  silicates.  Both  rocks 
analyzed  carried  a  trace  of  chlorine.  It  will  be  seen  that  the  rock  from  the 
central  mass  carries  a  higher  percentage  of  all  bases,  except  potash,  than 
the  narrow  dike.  The  latter  probably  represents  fairly  well  the  contact 
rocks  of  the  larger  dikes. 

It  is  possible  that  in  a  careful  study  of  these  rocks  with  reference  to 
the  development  of  crystallization  it  might  be  shown  that  the  ferro-magne- 
sian minerals  exhibited  a  tendency  to  segregate  in  the  central  or  less  rapidly 
crystallizing  portion  of  the  dike,  due  to  differentiation  in  the  chemical 
composition  of  the  molten  lava.  It  may  be  well  to  mention  here  that  in 
connection  with  these  miles  of  porphyry  dikes  there  are  no  evidences  of 
any  recent  volcanic  action.  The  granite-porphyries  and  the  rhyolites  seem 
to  be  wholly  independent  of  each  other  as  regards  their  mode  of  occurrence 
and  their  loci  of  eruption. 


CHAPTEE    VIII. 
TERTIARY  AND  POST-TERTIARY  VOLCANIC  ROCKS. 

Eureka  a  Volcanic  Center.— In  the  Eureka  District  the  recent  volcanic  erup- 
tions play  a  far  more  important  part  than  the  granites  and  porphyries  just 
described.  They  occur  in  much  larger  masses,  cover  more  extensive  areas, 
and  are  more  widely  distributed  over  the  district.  While  the  older  crystal- 
line rocks  have  exerted  little  influence  upon  the  surface  features  of  the 
country,  the  volcanic  rocks  have  greatly  modified  its  topographical  outlines, 
have  built  up  isolated  mountains,  broad  table-lauds,  and  numerous  small 
hills,  and  in  coming  to  the  surface  have  disturbed  and  broken  up  sedi- 
mentary formations,  greatly  complicating  geological  structure.  Moreover, 
the  volcanic  rocks  are  of  special  interest  from  an  economic  point  of  view, 
owing  to  their  intimate  geological  connection  with  the  argentiferous  lead 
deposits  occurring  in  the  adjoining  Paleozoic  rocks.  As  the  Eureka  Moun- 
tains are  surrounded  on  nearly  all  sides  by  the  characteristic  broad  valleys 
of  the  Nevada  plateau,  this  volcanic  region  occupies  a  somewhat  isolated 
position  with  reference  to  the  neighboring  ranges,  constituting  a  region 
quite  apart  from  all  other  centers  of  similar  eruptions,  yet  at  the  same  time 
bearing  the  closest  resemblance  in  the  nature  of  its  extra vasated  material 
to  many  other  localities  in  the  Great  Basin.  Nowhere  in  the  district  have 
the  accumulated  lavas  attained  any  great  elevation  above  the  surround- 
ing mountains,  but  as  regards  mode  of  occurrence,  peculiarities  of  distri- 
bution, and  varieties  of  modification  they  offer  a  wide  field  for  investigation. 
There  is  no  such  piling  up  of  enormous  masses  of  erupted  material  as  in 
the  Washoe  District,  and  no  such  opportunity  for  an  investigation  of  the 
more  coarsely  crystalline  rocks,  but,  on  the  other  hand,  the  relationship 
between  the  extrusive  lavas  and  the  uplifted  blocks  of  Paleozoic  strata  is 
better  shown  than  in  any  other  carefully  studied  area  in  the  Great  Basin. 

230 


AGE  OF  ERUPTION.  231 

Distribution  of  Extrusive  Lavas.— In  regard  to  the  distribution  of  volcanic 
rocks  in  the  Eureka  District  it  will  be  noticed  by  reference  to  the  map  that 
there  are  none  in  that  part  of  the  Diamond  Range  which  comes  within  the 
area  of  the  map.  Although  the  southern  end  is  completely  cut  off  by  recent 
lavas  from  the  mountain  block  of  County  Peak,  nowhere  do  they  seem  to 
penetrate  into  the  range  itself.  In  the  southern  end  of  the  Pinon  Range  on 
the  opposite  side  of  Diamond  Valley,  there  is  the  same  absence  of  lavas,  if 
we  except  a  small  outburst  northwest  of  The  Gate,  which  has  but  little  to  do 
with  the  main  body  of  the  Eureka  Mountains.  The  Mahogany  Hills  west 
and  north  of  Dry  Lake  furnish  one  or  two  small  exposures  of  rhyolite  along 
lines  of  faulting  111  limestone,  notably  at  the  head  of  Brown's  Canyon,  but 
they  are  of  110  special  geological  significance.  In  the  Fish  Creek  Mountains 
no  outbreaks  of  extrusive  rocks  are  known  and  the  same  holds  true  of  the 
mountains  bordering  the  Spring  Valley  fault.  In  the  extreme  southeast 
comer  of  the  map  the  Cliff  Hills  come  in,  formed  of  basic  andesites  similar 
to  those  of  Richmond  Mountain,  but  they  lie  wholly  beyond  the  borders  of 
the  Eureka  Mountains.  This  confines  the  area  of  the  principal  volcanic 
extravasations  either  to  the  region  east  of  the  summit  of  Prospect  Ridge  or 
to  the  country  encircling  the  southern  end  of  that  ridge,  where  it  sinks  below 
Fish  Creek  Valley.  Nearly  every  type  and  many  of  the  varieties  of  vol- 
canic rocks  found  upon  the  Nevada  plateau  have  been  erupted  within  these 
restricted  limits  of  the  Eureka  District.  Indeed,  the  region  furnishes  many 
rocks  which  may  be  taken  as  typical  of  a  broad  area  of  country  lying 
between  the  Wasatch  and  the  Sierra  Nevada  ranges. 

Age  of  Eruptions.— It  should  be  clearly  understood  that  the  Eureka  Dis- 
trict, like  many  other  regions  of  central  Nevada,  offers  no  direct  evidence 
as  to  the  age  of  volcanic  eruptions,  as  there  occur  no  sedimentary  forma- 
tions between  Upper  Coal-measure  limestones  and  recent  Pleistocene  de- 
posits. While  positive  evidence  may  be  wanting  as  to  their  precise  age, 
there  can  be  no  doubt  that  the  eruptions  took  place  subsequent  to  the 
dynamic  movements  which  brought  about  the  flexing,  folding,  and  mountain- 
building,  and  these,  while  it  may  not  have  been  demonstrated,  have  been 
assigned  upon  excellent  grounds  to  a  post-Jurassic  upheaval,  already  dis- 
cussed in  Chapter  II.  of  this  work.  Moreover,  that  these  orographic 


232  GEOLOGY  OF  THE  EUEEKA  DISTRICT. 

movements  were  followed  by  a  long  period  of  time  before  the  outbursts  of 
lavas  is  evident  by  the  amount  of  erosion  which  took  place  before  the  ex- 
trusion of  the  latter  rocks.  This  is  shown  by  the  position  of  the  lavas  in 
the  bottoms  of  deeply  eroded  canyons,  by  the  blocking  up  of  old  drainage 
channels  and  the  cutting  of  new  ones,  all  of  which  must  have  required 
considerable  time  to  be  accomplished  by  slow  geological  processes.  On 
the  other  hand,  based  upon  much  the  same  kind  of  evidence,  the  amount 
of  erosion  since  the  volcanic  period  seems  relatively  slight.  Although  evi- 
dence of  geological  age  of  these  lavas  may  be  difficult  to  obtain  over  the 
greater  part  of  the  Great  Basin,  wherever  such  proof  is  observed  it  always 
points  to  the  conclusion  that  the  eruptions  took  place  since  the  coming  in 
of  Tertiary  time  and  for  the  most  part  during  the  Pliocene  period.  Nearly 
all  geologists  who  have  examined  the  volcanic  areas  of  Nevada  and  Utah 
are  in  accord  with  the  opinion  that  they  belong  to  Tertiary  time.  In  the 
region  of  the  Montezuma  and  Kawsoh  ranges,  in  western  Nevada,  the  geol- 
ogists of  the  Fortieth  Parallel  Exploration1  have  described  the  cutting  by 
intrusive  acid  lavas  of  upturned  Miocene  strata  carrying  fresh-water  mol- 
luscan  shells  and  the  overlying  of  the  latter  beds  by  basaltic  flows.  Evi- 
dence has  also  accumulated  to  show  that  several  of  the  great  rhyolite  flows 
that  preceded  the  basalt  belong  to  the  Pliocene  epoch.  From  unques- 
tioned evidence  in  other  parts  of  the  Great  Basin  as  to  the  geological  posi- 
tion of  identical  lavas,  it  is  assumed  that  those  of  the  Eureka  District  were 
also  poured  out  during  the  Tertiary  age.  As  regards  the  duration  of  vol- 
canic activity  there  is  scarcely  any  evidence.  From  the  earliest  outbursts, 
consisting  of  homblende-andesites,  to  the  latest  eruptions  of  highly  glassy 
basalts,  volcanic  activity  may  have  extended  over  the  greater  part  of  late 
Tertiary  time.  That  this  activity  continued  at  varying  intervals  through- 
out a  long  period  seems  clear  from  the  amount  of  erosion,  which,  though 
relatively  not  excessive,  must  have  required  considerable  time.  There  is 
no  evidence  to  show  that  volcanic  energy  with  varying  intensity  may  not 
have  extended  through  Pliocene  well  on  into  Quaternary  time,  although 
there  is  no  reason  to  suppose  that  outbursts  of  basalt  have  taken  place 
within  what  may  be  called  historic  periods. 

1 U.  S.  Geol.  Explor.,  40th  Par.     Descriptive  Geology,  p.  771. 


CLASSIFi CATION  OF  LAVAS.  233 

Classification  of  Lavas.— On  the  geological  map  all  volcanic  rocks  have  been 
classed  under  the  following  heads :  Hornblende-andesite,  dacite,  rhyolite, 
pumice  and  tuff,  augite-andesite,1  and  basalt.  All  the  more  important  bodies 
of  lava  belong  to  some  one  of  these  sharply  denned  mineralogical  groups. 
It  must  be  borne  in  mind,  however,  that  several  of  these  types  of  igneous 
lavas  pass  by  insensible  gradations  from  one  into  another  and  it  is  not 
always  easy  to  decide  to  which  group  an  isolated  exposure  in  the  field  or  a 
hand  specimen  in  the  laboratory  properly  belongs.  All  division  lines  are 
more  or  less  arbitrary.  They  are  necessary  for  purposes  of  classification, 
although  they  may  not  exist  in  nature. 

To  inform  those  readers  who  have  not  kept  up  with  recent  ad- 
vances in  the  classification  of  igneous  rocks  and  at  the  same  time  to  pre- 
vent all  misunderstandings  as  to  the  use  of  terms,  a  brief  description 
will  be  given  of  the  physical  characteristics  of  each  group  of  lavas 
which  have  been  recognized  as  occurring  in  the  district.  Those  who 
desire  a  more  detailed  description  of  the  special  petrographical  features 
of  the  rocks  are  referred  to  the  report  of  Mr.  J.  P.  Iddings,  which  accom- 
panies this  work.  As  he  has  presented  the  results  of  a  most  thorough 
microscopical  investigation  of  the  material  collected  in  the  field,  in  order 
to  prevent  a  duplication  of  facts  much  that  might  properly  find  place  in 
these  pages  is  omitted,  and  only  such  data  are  employed  here  as  are  deemed 
necessary  to  make  this  chapter  complete  in  itself  and  to  bring  out  more 
clearly  the  important  facts  bearing  upon  the  geology  of  the  region. 

Hombiende-andesite.— Under  homblende-andesites  are  classed  those  vol- 
canic rocks  which  are  composed  mainly  of  triclinic  feldspars  and  horn- 
blendes as  essential  constituents.  Here  at  Eureka  the  fresh  unaltered 


1  Since  the  printing  of  the  atlas  (1883)  accompanying  this  Monograph,  the  pyroxenic  minerals 
of  the  andesito  of  Richmond  Mountain,  which  at  that  time  were  considered  to  be  augites,  have  beeu 
shown  to  consist  of  both  hypersthene  and  augite,  the  former  mineral  being  in  most  instances  largely  in 
excess.  The  rock,  therefore,  should  more  properly  be  designated  as  pyroxeue-andesite,  a  designation 
more  in  accord  with  the  nomenclature  now  generally  adopted  by  lithologists  for  similar  rocks  else- 
where. 

Throughout  this  volume  the  term  "pyroxeue-andesite"  will  be  used  to  designate  the  rocks  of 
Richmond  Mountain  and  Clift' Hills,  the  two  localities  colored  as  augite-andesite  on  the  atlas  sheets. 

See  Notes  on  the  volcanic  rocks  of  the  Groat  I '.a  sin.  Hague  and  Iddiugs:  Am.  Jour.  Sri..  June, 
1884,  p.  457. 


234  GEOLOGY  OF  THE  EUEEKA  DISTRICT. 

rock  .is  of  a  light  reddish-purple  color,  for  the  most  part  holocrystalline  in 
structure,  with  well  developed  secretions  of  the  two  principal  minerals, 
accompanied  by  varying  amounts  of  biotite.  The  black  bordered  horn- 
blende is  almost  exclusively  confined  to  macroscopic  individuals  which  are 
for  the  most  part  decomposed  into  chloritic  material,  frequently  giving  the 
altered  rocks  a  green  color.  Magnetite,  though  not  sufficiently  abundant 
to  be  regarded  as  an  essential  ingredient,  appears  to  be  evenly  disseminated 
throughout  the  groundmass,  and  quartz,  wanting  in  most  varieties,  serves  as 
an  accessory  mineral  in  certain  localities,  especially  in  the  more  acid  types. 
The  mica  gradually  comes  in  as  the  rock  becomes  more  and  more  acidic 
and  as  the  acidic  variety  of  this  group  predominates  over  the  basic  the 
mica  occurs  as  a  frequent  if  not  constant  constituent.  Hornblende-mica- 
andesite  covers  much  larger  areas  than  normal  horublende-andesite.  A 
characteristic  and  important  feature  of  the  hornbleude-andesite  of  this  dis- 
trict is  the  absence  of  all  pyroxene.  Very  little  of  the  horublende-andesite 
is  perfectly  fresh  and  most  of  it  has  undergone  a  considerable  decomposi- 
tion, changing  the  color  of  the  rock  to  light  shades  of  red  and  yellow, 
while  those  portions  which  are  most  altered  appear  nearly  white.  Opal 
and  chalcedony  as  secondary  products  are  by  no  means  uncommon  in  the 
more  altered  varieties. 

Andesitic  Peariiteu.— Nearly  every  occurrence  of  hornblende-andesite  is 
accompanied  by  more  or  less  extensive  outflows  of  andesitic  pearlites, 
which  so  far  as  their  mineral  composition  is  concerned  are  quite  similar  to  and 
in  many  instances  identical  with  the  crystalline  types.  They  are  in  general 
more  acidic  in  composition  than  the  hornblende-mica-andesites,  carry  fewer 
well  developed  crystals,  and  in  place  of  the  holocrystalline  structure  are 
rich  in  glass  base  with  microcrystalline  secretions  disseminated  through  it. 
It  is  the  almost  infinite  varieties  of  this  glass  base  which  give  to  these 
pearlitic  rocks  their  varying  physical  habit,  color,  luster,  and  density. 
Owing  to  their  more  acidic  character,  quartz  becomes  more  frequent,  but  is 
shown  rather  in  macroscopic  secretions  than  in  minute  grains  scattered 
through  the  groundmass.  Sanidine,  wanting  in  the  normal  varieties, 
may  occur,  although  as  a  nonessential  mineral,  while  biotite  comes  promi- 
nently to  the  front  and  to  the  eye  appears  as  the  most  abundant  macro- 


ANDESITIC  PEAELITES.  235 

scopic  mineral,  and  relates  the  pearlites  more  closely  to  the  hornblende- 
mica-andesites  than  to  the  more  basic  group.  A  variety  of  these  pearlites 
exceptionally  rich  in  glass  is  characterized  by  the  appearance  of  hyper- 
sthene.  As  it  is  one  of  the  most  basic  of  this  group  of  andesites  it  will  be 
discussed  farther  on  in  this  chapter.  Many  of  the  varieties  possess  a  dense 
vitreous  texture,  breaking  readily  under  a  hammer-blow,  while  others  are 
more  or  less  pumiceous,  crumbling  easily  under  atmospheric  agencies. 

The  largest  body  of  hornblende-andesite  occurs  as  a  fissure  eruption 
along  the  Hoosac  fault,  the  lavas  coming  to  the  surface  just  to  the  south  of 
the  junction  of  the  Ruby  Hill  branch  with  the  main  fault  and  extending 
southward  till  lost  beneath  rhyolitic  flows.  Like  most  acidic  rocks  these 
extravasated  lavas  have  not  spread  out  over  large  areas,  but  have  piled  up 
in  irregular  rounded  hills,  the  highest  reaching  an  elevation  of  500  feet 
above  the  base  of  the  limestones  along  the  fault  line  in  the  valley.  For 
the  greater  part  of  the  distance  along  this  fissure  these  lavas  have  under- 
gone more  or  less  alteration,  due  to  solfataric  action,  kaolinization  taking 
place  with  the  formation  of  secondary  minerals.  Comparatively  fresh  rocks 
not  far  from  the  fault  are  still  found  northeast  of  Hoosac  Mountain  in  the 
larger  and  least  altered  bodies.  Associated  with  the  more  crystalline  types 
occur  excellent  exposures  of  andesitic  glasses  and  pearlites,  products  ot 
more  rapid  cooling  of  the  same  magma  under  slightly  different  physical 
conditions.  Other  localities  of  hornblende-mica-andesite  with  the  accom- 
panying pearlites  closely  resembling  each  other  in  manner  of  occurrence 
and  mineral  composition  are  found  at  the  southern  end  of  Carbon  Ridge, 
in  the  neighborhood  of  South  Hill,  at  Spring  Valley,  and  near  Dry  Lake. 
In  all  of  the  four  latter  localities  hornblende-andesite  and  the  horn- 
blende-mica-andesite  occupy  positions  quite  inferior  to  the  glassy  varieties, 
so  far  as  the  amount  of  extravasated  lava  is  concerned,  but  it  is  by  no 
means  easy,  owing  to  insensible  gradations,  to  draw  a  sharp  line  between 
the  crystalline  and  glassy  types.  At  the  first  two  localities  they  occur, 
breaking  out  at  the  southern  base  of  the  upturned  longitudinal  ridges 
of  sedimentary  strata.  In  Spring  Valley  and  along  tKe  Lookout  fault, 
where  the  lavas  penetrate  the  mountains  on  the  west  side  of  Prospect  Ridge, 
the  glassy  varieties  have  poured  out  in  relatively  large  masses,  pearlites 


236  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

being  tne  prevailing  rock.  They  form  gentle  spurs  200  feet  in  height,  rest- 
ing against  the  sedimentary  ridges.  An  interesting  transition  rock  occurs 
here  which  unites  the  characteristic  microcrystalline  hornblende-mica- 
andesite  with  fine  examples  of  pearlite.  At  Dry  Lake,  while  the  petro- 
graphical  features  are  much  the  same,  with  similar  transitions  and  variations, 
the  geological  occurrence  is  somewhat  different,  the  locality  being  quits 
remote  from  the  others  and  lying  far  to  the  west  of  Prospect  Ridge  and 
the  principal  lines  of  north  and  south  faulting.  A  suite  of  specimens  from 
any  one  of  these  localities  could  hardly  be  distinguished  from  those  of 
others  either  in  physical  features  or  composition.  Identical  transition  prod- 
ucts, both  as  regards  degree  of  crystallization  and  mineral  variation,  are 
recognized  from  all  of  them.  They  show  the  same  order  of  succession  and 
the  same  sequence  of  geological  events.  The  petrographical  features  of 
these  nearly  identical  series  of  lavas  will  be  found  described  by  Mr.  Iddings 
in  the  chapter  already  referred  to. 

Dacite.— This  rock  is  a  variety  of  andesite,  in  which  secretions  of  quartz 
play  the  part  of  an  essential  mineral,  and  is  intimately  related  in  mineral 
composition  and  structural  habit  to  hornblende-mica-andesite.  All  the 
occurrences  at  Eureka  are  very  similar  in  physical  characteristics,  possess- 
ing a  light  ash-gray  color,  a  hackly  fracture,  and  a  rough,  pumice-like 
texture  which  relate  them  closely  to  certain  varieties  of  rhyolite  with  which 
in  the  field  they  are  frequently  associated  and  from  which  they  are  not 
easily  separated,  either  in  their  mode  of  occurrence  or  lithological  appear- 
ance. The  feldspars  are  nearly  all  plagioclase  and  probably  largely  oligo- 
clase.  Together  with  the  larger  secretions  of  quartz  occur  numerous  thin 
lamina;  of  biotite,  in  amount  greatly  in  excess  of  that  usually  found  in  horn- 
blende-andesite.  Although  of  less  frequent  occurrence,  occasional  hyper- 
sthene  crystals,  similar  to  those  found  in  the  andesitic  pearlite,  may  be 
recognized  in  the  dacites  and  serve  to  show  the  dying  out  of  ferro-mag- 
nesian  minerals  in  the  more  acidic  lavas.  These  glassy  dacites  are  closely 
related  to  the  andesitic  pearlites,  but  occupy  much  smaller  areas,  their  chief 
interest  lying  in  the  fact  that  they  represent  transition  products  from  the 
andesitic  to  the  rhyolitic  lavas,  sometimes  associated  with  one,  sometimes 
with  the  other,  and  not  infreauently  as  lava  flows  connecting  them 


DACITE.  21)7 

both,  in  much  the  same  way  as  the  more  crystalline  dacites  occur 
as  transition  products  between  the  crystalline  varieties  of  hornblende-mica- 
andesite  and  rhyolite.  Near  the  entrance  to  Sierra  Canyon,  011  botli 
sides  of  the  road,  there  is  exposed  a  characteristic  variety  of  gray 
dacite,  and  at  Dry  Lake  and  South  Hill  they  occur  with  andesitic  eruptions, 
but  only  in  obscure  low  ridges  and  knolls,  breaking  through  the  Nevada 
limestone  of  the  Devonian.  Again  some  varieties  of  dacite  are  closely 
associated  geologically  with  rhyolitic  pumices  and  tuffs,  but  differ  from 
them  petrographically  in  having  a  predominance  of  triclinic  instead  of  mono- 
clinic  feldspars. 

Rhyolite.- The  essential  components  of  this  natural  group  are  restricted 
to  orthoclase  and  quartz.  Usually  they  carry  more  or  less  triclinic  feld- 
spars, in  some  cases  almost  equaling  the  monoclinic  form,  but  they  are 
rarely  developed  in  as  large  individuals  as  the  orthoclase.  Biotite  as  an 
accessory  mineral  may  be  present  in  varying  amounts,  but  is  quite  as  likely 
to  be  wholly  wanting.  In  chemical  composition  they  form  the  most  acid 
of  all  natural  groups  into  which  the  lavas  have  been  divided.  In  color  and 
texture  no  rock  surpasses  the  rhyolite  in  the  endless  modifications  which  it 
undergoes  even  within  very  limited  areas.  In  crystalline  structure  it  may 
vary  from  a  rock  possessing  a  holocrystalline  groundmass,  with  or  without 
large  macroscopic  secretions  of  the  essential  minerals,  to  one  almost  wholly 
made  up  of  glass.  Whether  the  rhyolite  is  crystalline  or  in  large  degree 
composed  of  glass,  the  sanidines  occur  in  well  developed  crystals,  frequently 
presenting  the  brilliant  iridescent  hues  so  often  observed  elsewhere  through- 
out the  Great  Basin:  The  quartz  occurs  both  as  dihexahedral  crystals  and 
dark  gray  and  black  angular  grains  which  stand  out  in  strong  contrast  to 
the  prevailing  light  tints  of  the  inclosing  groundmass. 

At  Eureka,  where  acid  lavas  are  singularly  well  developed,  among 
the  many  extrusions  of  rhyolite  occur  two  principal  varieties  which  cover 
large  areas  and  embrace  the  greater  part  of  the  outbursts,  and  for  the  pur- 
poses of  the  present  chapter  may  be  designated  by  local  names:  one,  the 
Rescue  Canyon  rhyolite,  the  other,  the  Pinto  Peak  rhyolite.  In  mineral 
and  chemical  composition  they  are  closely  allied.  The  Rescue  Canyon 
rhyolite  when  fresh  has  a  decidedly  red  color  due  to  a  considerable  amount 


238  GEOLOGY  OF  THE  EUEEKA  DISTE1CT. 

of  iron  oxide  iu  the  groundmass.  It  occurs  only  as  a  fissure  eruption  along 
the  Rescue  fault,  the  brilliancy  of  coloring  causing  it  to  stand  out  promi- 
nently in  contrast  with  the  inclosing  dark  gray  limestones.  It  is  largely 
composed  of  glass  base  in  which  are  porphyritically  imbedded  exception- 
ally brilliant  grains  of  dark  quartz  and  tabular  crystals  of  sanidine.  The 
Pinto  Peak  rhyolite,  on  the  other  hand,  is  characterized  by  a  more  crystal- 
line groundmass,  much  of  it  being  holocrystalline.  It  is  iu  general  lighter 
in  color,  more  varied  in  tint,  carries  less  iron,  and  is  almost  wholly 
free  from  ferro-magnesian  secretions.  In  places  it  disintegrates  readily 
into  loose  quartz  grains  and  feldspathic  fragments.  The  name  is  taken 
from  Pinto  Peak,  a  prominent  elevation,  made  up  wholly  of  rhyolitic 
accumulations,  lying  between  Spring  Hill  and  Carbon  Ridge,  just  east  of  the 
Hoosac  fault.  Similar  rhyolites,  singularly  uniform  in  composition  and 

» 

crystalline  structure,  extend  without  a  break  in  their  continuity  the  entire 
distance  from  Pinto  Peak  to  Gray  Fox  Peak.  Nearly  all  the  lesser  out- 
bursts of  rhyolite  scattered  over  the  district  and  breaking  out  along  fault 
planes  belong  to  the  Pinto  Peak  variety.  It  is  characteristic  of  many  vol- 
canic centers  in  the  Great  Basin  as  well  as  Eureka,  and  may  be  considered  as 
a  typical  rock  over  large  areas. 

Pumice  and  Tuff.— All  rocks  placed  under  this  head  are  closely  allied  to 
the  rhyolites  in  mineral  and  chemical  composition  and  belong  to  the  same 
natural  group.  The  rhyolites  and  pumices  break  out  under  very  similar 
geological  conditions  and  frequently  pass  from  one  into  the  other,  even 
more  readily  than  the  transition  between  the  crystalline  andesites  and 
glassy  pearlites.  They  cover  much  larger  areas  than  the  corresponding 
andesitic  rocks  and  in  their  field  occurrence  offer  such  striking  contrast  to 
the  more  compact  rhyolite  that  it  is  thought  best  to  separate  them,  more 
especially  as  they  exhibit  definite  characteristics  sufficient  to  group  them  on 
the  map  by  themselves,  and  in  several  localities  the  erupted  material  con- 
sists wholly  of  pumices  and  tuffs.  Transitions  from  normal  rhyolites  into 
pumices  are  admirably  shown  along  the  base  of  Purple  Mountain.  The 
mass  of  the  mountain  rises  400  feet  above  the  valley  and  is  formed  of  a 
characteristic  rhyolite,  while  the  base  spreads  out  in  a  great  variety  of 


PYKQXESE  ASDES1TE.  239 

pumices  and  tuffs,  which  stretch  eastward  beneath  the  town  of  Eureka  as 
far  as  Richmond  Mountain. 

Similar  transitions  may  also  be  seen  in  the  neighborhood  of  Pinto 
Peak,  although  on  a  less  extensive  scale.  It  seems  impossible  for  any 
region  to  exhibit  a  finer  display  of  pumices  and  tuffs  than  those  occupying 
the  basin  between  County  Peak  and  Richmond  Mountain.  Here,  in  the 
neighborhood  of  Hornitos  Cone,  a  symmetrical  hill  of  tuff  400  feet  in 
height,  these  pumices  are  shown  with  every  possible  variation  in  color  and 
texture,  the  results  of  alteration  produced  by  the  breaking  through  of 
basaltic  masses.  The  color  and  density  have  undergone  marked  changes, 
but  the  mineral  development  remains  much  the  same  as  in  the  normal 
rhyolite.  Nothing  could  surpass  the  abrupt  changes  in  physical  habit 
which  these  rocks  undergo. 

Pyroxene-andesite  (Augite-andesite).— Under  pyroxene-aiidesite  are  included 
all  those  volcanic  rocks  whose  essential  constituents  consist  of  triclinic  feld- 
spars and  pyroxenes,  the  rock  differing  fundamentally  from  the  hornblende- 
andesite  in  having  the  hornblende  replaced  by  some  form  of  pyroxene, 
usually  a  mingling  of  both  hypersthene  and  augite.  Hyperstheue  in  most 
rocks  of  this  group  surpasses  the  augite  in  amount  and  in  certain  localities 
predominates  to  such  a  degree  that  the  rock  might  properly  be  classed  as 
hypersthene-andesite.  Whether  there  exists  any  large  body  of  extrusive 
lava  in  the  Great  Basin  retaining  the  andesitic  habit,  in  which  the  augite  is 
the  prevailing  mineral,  to  the  exclusion  of  hypersthene  and  without  the 
accompaniment  of  olivine,  is  a  matter  of  some  doubt.  The  rock  is  rich  in 
magnetite,  disseminated  throughout  the  mass,  the  mineral  playing  a  far 
more  important  part  than  in  hornblende-andesites.  In  addition  to  the  essen- 
tial mineral  constituents  which  make  up  the  rock,  both  biotite  and  black- 
bordered  hornblende  have  been  identified  in  the  pyroxene-andesites  from 
several  localities  in  considerable  quantity.  Richmond  Mountain  is  the  only 
body  of  pyroxene-andesite  in  the  Eureka  District.  It  is  well  represented 
in  the  Cliff  Hills  just  south  of  the  Fish  Creek  basin,  the  northern  end  of 
which  is  shown  on  atlas  sheet  xn.  It  covers  such  an  extensive  area,  present- 
ing not  only  an  important  feature  of  the  lavas  of  this  region,  but  is  such  a 
typical  rock  of  many  other  localities,  that  it  requires  to  be  described  in  detail. 


240  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

Richmond  Mountain.— The  mountain  lies  in  a  region  of  profound  disturb- 
ance and  dislocation.  Immediately  to  the  west  the  depressed  block  of 
Spring  Hill  sinks  beneath  the  plain,  while  to  the  south  the  broad  elevated 
mass  of  County  Peak  rises  abruptly  above  the  lavas.  The  Pinto  fault  passes 
beneath  Richmond  Mountain  and  apparently  connects  with  the  Rescue 
fault,  the  great,  line  of  displacement  which  separates  the  County  Peak 
block  from  the  Diamond  Range,  but  all  structural  features  are  obliterated 
by  the  pyroxene-andesite  lava  flows.  The  culminating  point  of  Richmond 
Mountain,  situated  near  its  southern  end,  attains  an  elevation  of  nearly 
2,000  feet  above  Diamond  Valley.  An  abrupt  wall,  800  feet  in  height, 
forms  the  southern  end,  and  from  its  summit  the  mountain  falls  away  to 
the  north  for  nearly  three  miles,  with  an  average  slope  of  about  14°. 
Across  its  broadest  expansion,  in  an  east  and  west  direction,  the  mountain 
measures  three  miles.  For  such  an  accumulated  pile  of  lavas  it  presents  a 
uniform,  monotonous  appearance,  relieved  by  occasional  shallow  drainage 
depressions  flowing  northward,  inclined  with  the  natural  slopes  of  its  lava 
ridges.  Its  geological  position  with  reference  to  the  Carboniferous  beds 
of  the  Diamond  Range  on  the  east,  the  Devonian  on  the  south,  and  the 
Silurian  and  Cambrian  beds  on  the  west  is  shown  in  cross-section  A-B, 
atlas  sheet  xm. 

Richmond  Mountain  is  almost  wholly  made  up  of  pyroxene-andesite, 
the  prevailing  colors  of  which  are  dark  grayish  purple  varying  to 
bluish  black.  In  crystalline  structure  the  rock  varies  from  a  micro- 
crystalline  groundmass  to  one  rich  in  glass  base,  porphyritic  crystals 
of  light  colored  feldspars  characterizing  the  rock  through  all  degrees  of 
crystallization.  This  range  in  crystallization  produces  marked  variations 
in  physical  features,  the  lavas  changing  within  short  distances  from  a 
highly  vesicular  rock  with  angular  fracture  to  a  compact  one  weathering 
with  rounded  outlines.  Varying  proportions  of  the  porphyritic  constitu- 
ents are  found  in  all  the  rocks  from  the  holocrystalline  to  those  rich  in 
glass  base.  Hypersthene  is  the  prevailing  pyroxenic  mineral,  always 
accompanied,  however,  by  more  or  less  augite.  The  feldspars  are  anorthite 
and  labradorite.  In  addition  to  the  essential  minerals,  well  developed 
porphyritic  crystals  of  hornblende  occur  in  the  less  basic  lava,  but  nowhere 


RICHMOND  MOUNTAIN.  241 

as  shown  by  the  microscope  do  they  enter  into  the  composition  of  the 
groundmass.  Associated  with  the  hornblendes  are  a  few  flakes  of  biotite, 
the  latter  mineral  occasionally  appearing  scattered  through  the  rock  with- 
out the  presence  of  the  former.  It  is  this  association  of  hornblende  and 
biotite  in  the  pyroxene-audesite  that  relates  it  to  the  earlier  hornblende- 
audesite.  These  relatively  acid  lavas  are  well  shown  in  the  neighbor- 
hood of  Trail  Hill.  In  all  these  rocks  of  Richmond  Mountain  the 
groundmass  is  made  up  of  innumerable  lath-shaped  feldspars  and  micro- 
lites  of  pyroxene,  producing  that  peculiar  felt-like  structure  first  described 
by  Prof.  Zirkel  and  since  recognized  by  others  as  a  characteristic  of 
pyroxene-andesites.  Two  typical  varieties  occur  here,  extreme  forms 
of  the  same  lava :  one,  a  rough,  porous  rock,  dark  purple  in  color  and 
having  what  has  frequently  been  called  a  trachytic  texture ;  the  other,  a 
compact  rock,  bluish  black  in  color  and  possessing  an  oily  resinous  luster 
which  has  been  described  as  a  characteristic  of  many  pyroxene-andesites 
elsewhere.  Both  of  these  sharply  contrasted  rocks  pass  by  insensible  transi- 
tions, the  one  into  the  other,  preventing  the  tracing  out  of  separate  flows 
in  the  field.  There  is,  however,  this  marked  peculiarity  between  them: 
the  former  has  a  laminated  and  fissile  appearance,  whereas  the  latter 
nowhere  exhibits  any  tendency  to  such  lines  of  parting. 

Owing  to  the  geological  and  petrographical  importance  of  the  Rich- 
mond Mountain  rocks,  Mr.  Iddings  has  devoted  much  time  to  an  investi- 
gation of  the  lime-soda-feldspars  and  ferro-magnesian-silicates,  the  results  of 
which  will  be  found  in  detail  in  his  chapter  on  microscopical  petrography. 
Mr  Iddings  has  determined  anorthite  by  its  extinction  angles  and  other 
optical  properties  as  one  of  the  prevailing  feldspars  which  at  the  time  of  his 
work  was  the  first  recognition  of  this  species  as  an  essential  constituent  of 
the  volcanic  rocks  of  the  Great  Basin. 

Within  recent  years  investigations  have  demonstrated  that  hypersthene 
is  the  prevailing  ferro-magnesian  silicate  of  many  pyroxeue-andesites  in 
volcanic  regions  throughout  the  world.  The  importance  of  hypersthene 
as  an  essential  and  controlling  constituent  of  pyroxene-andesites  in  Colorado 
was  first  shown  by  Whitman  Cross.1  Soon  after  these  observations  were 

1  Bulletin  of  the  U.  S.  Geological  Survey,  No.  1,  1883. 
MON  XX 10 


GEOLOGY  OF  THE  EUREKA  DISTRICT. 

confirmed  and  extended  by  an  examination  of  the  lavas  from  the  volcanoes 
of  the  Pacific  Coast'  and  those  of  the  Great  Basin.2  In  the  investigations  of 
these  latter,  the  pyroxene-andesites  of  Richmond  Mountain  played  an  in- 
teresting part,  all  the  more  important  as  a  large  suite  of  rocks  from  one 
locality  whose  field  relations  were  known  were  subjected  to  a  most  careful 
mineralogies!  study.  The  isolation  of  the  hypersthene  from  augite  was 
accomplished  by  means  of  a  solution  of  cadmium-boro-tungstate  having  a 
specific  gravity  of  3 '3  9.  Repeated  treatment  with  the  solution  yielded  a 
brown  hyperstheue  carrying  a  small  amount  of  green  augite.  Under  the 
microscope  the  former  proved  to  be  orthorhombic  in  form  and  strongly 
pleochroic,  the  latter  monoclinic  and  without  pleochroism.  In  the  greater 
part  of  the  pyroxene-andesites  of  Richmond  Mountain  the  hypersthene  was 
always  found  to  be  in  excess  of  the  augite,  the  prevailing  minerals  being 
hypersthene  and  anorthite,  accompanied  by  labradorite  and  possibly  other 
plagioclase  feldspars,  together  with  varying  amounts  of  augite  and 
magnetite. 

Basalts.— Under  basalts  are  included  those  volcanic  rocks  which  have  for 
their  essential  ingredients  plagioclase  augite  and  magnetite.  Olivine,  which 
occurs  as  a  common  accompaniment  in  varying  proportions,  in  many  vari- 
eties, is,  however,  too  frequently  wanting  to  be  rigidly  regarded  as  an 
essential  constituent.  The  basalts  form  the  most  basic  of  all  natural  groups 
into  which  the  volcanic  lavas  have  been  divided.  At  Eureka  they  present, 
for  the  wide  field  which  they  cover  and  the  great  number  of  their  extru- 
sions, a  uniform  appearance,  and,  although  characterized  by  a  large  amount 
of  glass  base,  may  be  regarded  as  typical  of  many  localities  in  Nevada. 
It  is  fine  grained  and  compact,  frequently  passing  into  vesicular  forms,  with 
but  few  macroscopic  secretions,  and  by  far  the  greater  part  of  it  grayish 
black  in  color.  Olivine  occurs  in  large  grains  and  in  such  quantities  as 
occasionally  to  modify  the  external  character  of  the  rock ;  yet  over  broad 
areas  it  is  wholly  wanting,  the  microscope  failing  to  detect  its  presence  in 

1  Notes  on  the  Volcanoes  of  Northern  California,  Oregon,  and  Washington  Territory.  Hague 
and  Iddings.  Am.  Jour.  Sci.,  September,  1883,  vol.  26,  pp.  222-235. 

'Notes  on  the  Volcanic  Bocks  of  the  Great  Basin.  Hague  and  Iddiugs.  Am.  Jour.  Sci., 
Jane,  1884,  vol.  27,  pp.  453-463. 


DISTEIBDTION  OF  LAVAS.  243 

many  thin  sections.  Hypersthene  is  wanting  in  the  normal  basalts,  and  if 
present  is  only  recognized  in  the  intermediate  rocks  between  typical  pyrox- 
ene-andesite  and  basalts.  An  exceptionally  fine  display  of  these  intermedi- 
ate rocks  makes  this  group  of  special  geological  importance  at  Eui-eka.  A 
discussion  of  these  transition  rocks  is  reserved  till  later  in  the  chapter, 
when  treating  of  the  relations  of  the  different  groups  to  each  other. 

Manner  of  Occurrence  of  Volcanic  Lavas.— In  the    Eureka    District    there  are  11O 

grand  craters  through  which  the  greater  part  of  the  lavas  reached  the  surface 
and  from  which  volcanic  energy  receding  from  centers  of  igneous  action 
gradually  decreased  in  intensity  and  finally  died  out  altogether.  On  the 
contrary,  the  igneous  rocks  consist  for  the  most  part  of  extrusive  lavas  that 
have  poured  out  through  numerous  vents  scattered  over  the  volcanic  area, 
many  of  the  outbursts  being  very  limited  in  extent.  At  first  sight  it  might 
seem  impossible  to  recognize  any  order  in  their  distribution,  so  irregularly 
do  they  appear  to  break  out  in  most  unexpected  places.  Further  observa- 
tion, however,  shows  how  dependent  these  outbursts  are  upon  the  pre- 
existing orographic  structure  a  knowledge  of  which  is  absolutely  necessary 
to  a  thorough  understanding  of  the  volcanic  phenomena. 

As  regards  their  mode  of  occurrence,  all  the  lavas  may  be  classed  under 
four  heads:  first,  and  most  important,  they  break  out  along  the  three  great 
meridional  and  approximately  parallel  lines  of  displacement,  the  Hoosac, 
Pinto,  and  Rescue  faults;  second,  they  border  and  almost  completely 
encircle  the  large  uplifted  masses  of  sedimentary  strata,  like  the  Silverado  and 
County  Peak  block;  third,  they  occur  in  numerous  narrow  dikes  penetrat- 
ing the  limestones,  but  for  the  most  part  confined  to  Prospect  Ridge,  and, 
fourth,  they  occur  in  one  or  two  relatively  large  bodies,  notably  Richmond 
Mountain  and  Pinto  Peak,  along  lines  of  displacement  already  mentioned. 
Richmond  Mountain  is  situated  at  the  junction  of  the  Pinto  and  Rescue 
faults,  while  the  lava  of  Pinto  Peak  has  been  piled  up  along  an  oblique 
fault,  which  runs  from  the  Hoosac  to  the  Pinto  fault,  and  which  separates 
Spring  Hill  from  Carbon  Ridge.  It  is  along  the  lines  of  these  latter 
faults  that  the  most  powerful  volcanic  activity  has  been  displayed.  As 
described  elsewhere,  these  two  faults  are  situated  respectively  on  the  east 
and  west  sides  of  the  depressed  block  of  Carboniferous  rocks  lying  between 


244  CKOLOGY  OF  THP]  EUEEKA  DISTRICT. 

the  Cambrian  and  Silurian  of  Prospect  Ridge  on  the  one  side,  and  the 
Silurian  and  Devonian  of  County  Peak  and  Silverado  Mountain  on  the 
other.  These  profound  faults,  as  already  described,  show  nearly  2£  miles 
of  vertical  displacement.  Fissures  accompany  these  faults  and  through 
them  vast  masses  of  lavas  have  reached  the  surface  and  poured  out  along 
both  sides  of  the  fault  planes.  In  places  the  lavas  are  found  only  in  narrow 
belts  without  any  great  accumulation  of  material  and  in  others  they  are 
piled  up  in  rounded  hills  and  knolls  of  irregular  outlines,  concealing  every- 
thing beneath  them  for  long  distances.  In  general  acid  lavas  accumulate 
near  their  source  of  eruption  while  basic  lavas,  owing  to  their  greater  fluid- 
ity, show  a  tendency  to  flow  from  their  vents  in  broad  masses.  Along  the 
Hoosac  fault  from  Fish  Creek  Valley  to  its  junction  with  the  Ruby  Hill 
fault,  a  continuous  body  of  acid  lavas,  either  rhyolites  or  hornblende-aude- 
sites,  follow  the  course  of  the  fault,  but  beyond  their  junction  no  lavas  come 
to  the  surface  along  the  Hoosac,  although  they  persistently  follow  the  course 
of  the  Ruby  Hill  fault  as  described  in  detail  in  the  chapter  devoted  to  the 
discussion  of  the  ore  deposits  of  the  District.  Along  the  latter  fault  the 
rhyolites  do  not  form  a  continuous  surface  overflow,  but  break  out  in 
isolated  knolls  all  the  way  from  its  junction  with  the  Hoosac  to  the  Jackson 
fault,  beyond  which,  on  Ruby  Hill,  the  lavas  never  reach  the  surface, 
although  their  presence  is  shown  by  underground  mining  galleries.  West- 
ward from  the  Hoosac  fault  volcanic  energy  slowly  died  out. 

Following  the  trend  of  the  Pinto  fault  from  the  southern  end  of  the 
mountains,  rhyolitic  pumices  and  tuffs  define  the  general  course  of  displace- 
ment all  the  way  to  Dome  Mountain.  This  light  porous  material  has,  by 
gradually  welling  out  along  the  fissure,  heaped  up  bosses  and  knolls  of  vol- 
canic products,  but  owing  to  their  peculiar  physical  habit  they  have  eroded 
more  easily  than  the  denser  rocks  and  present  a  more  broken,  undulating 
surface.  Isolated  outbursts  of  pumices  occur,  penetrating  the  sedimentary 
strata  on  both  sides  of  the  fault,  and  along  Wood  Valley  extend  far  back 
into  the  Devonian  limestones.  Northward  of  Dome  Mountain  the  pumices 
and  tuffs  again  come  in,  but  are  finally  lost  beneath  the  imposing  mass  of 
pyroxene-andesites  of  Richmond  Mountain.  Along  the  Pinto  fault  only 


FISSUKE  ERUPTIONS.  245 

one  occurrence  of  andesite  is  known,  but  on  the  other  hand  numerous 
rounded  bosses  of  basalt  occur  on  both  sides  of  the  fault  line. 

The  Rescue  fault,  as  regards  the  amount  of  displacement,  is  a  less 
profound  one  than  the  others  just  mentioned,  but  it  is  as  sharply  outlined, 
and  owing  to  the  pre-Tertiary  erosion  of  Rescue  Canyon  exhibits  quite  as 
striking  an  occurrence  of  volcanic  outbursts.  The  erupted  material  does 
not  follow  the  entire  line  of  the  fault,  but  is  confined  to  its  southern  end, 
from  Island  Mountain  southward,  crossing  Silverado  Canyon  and  following 
along  the  east  side  of  Rescue  Canyon.  It  presents  a  most  remarkable  body 
of  erupted  material  nearly  2  miles  in  length,  seldom  exceeding  200  or  300 
feet  in  width.  Starting  in  at  the  base  of  the  escarpment  upon  the  east  side  of 
Island  Mountain,  along  which  runs  the  fault  plane,  it  descends  gradually 
for  750  feet  to  the  open  country  of  Fish  Creek  Valley.  The  extravasated 
material  is  wholly  composed  of  rhyolite  so  uniform  in  appearance  and  com- 
position, and  so  characteristic  of  the  region,  as  to  well  deserve  the  designa- 
tion of  the  Rescue  Canyon  rhyolite.  Rhyolitic  pumices  and  tuffs,  which 
occupy  the  valley  near  the  base  of  the  mountains,  conceal  the  denser  rock 
and  obscure  all  structural  features. 

Subordinate  to  the  eruptive  outbursts  along  these  three  great  fissures 
and  to  the  west  of  the  Hoosac  fault,  occur  two  other  narrow  belts  of 
igneous  rocks  similar  as  to  their  geological  position,  but  far  less  important 
as  lines  of  eruptive  energy.  They  are  found  on  the  west  side  of  the 
southern  end  of  Prospect  Ridge  and  penetrate  into  the  mountains  from  Fish 
Creek  Valley.  The  most  easterly  outbreak  occurs  along  Sierra  Valley. 
Just  west  of  it  in  Gray's  Canyon,  on  the  west  side  of  South  Hill,  lies  the 
second  line  of  lava  extrusions.  The  lava  thrown  out  along  these  secondary 
faults  is  restricted  in  amount,  bearing  some  relation  to  the  importance  of  the 
orographic  displacements. 

Closely  related  to  these  north  and  south  lines  of  eruption  occur  extrav- 
asated masses,  completely  surrounding  the  uplifted  blocks.  It  is  evident 
that  they  follow  lines  of  orographic  fractures,  more  or  less  profound, 
although  the  amount  of  displacement  can  seldom  be  determined.  In 
some  instances  it  is  quite  possible  to  estimate  the  faulting,  but  as  a  rule 
these  lines  of  east  and  west  orographic  fractures  are  completely  obscured 


246  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

by  recent  Quaternary  accumulations  or  else  buried  beneath  broad  masses 
of  lava.  These  latter  overflows  of  lava,  breaking  out  along  the  base  of 
the  escarpments,  follow  a  somewhat  sinuous  course,  yet  cling  most  per- 
sistently to  the  border  line  of  the  uplifted  area,  and  vary  greatly  in  the 
amount  of  extravasated  material  and  the  manner  in  which  they  pile  up  at 
the  surface  along  Ikies  of  fracture. 

The  most  striking  illustration  of  this  mode  of  occurrence  may  be 
seen  in  the  lavas  surrounding  the  County  Peak  and  Silverado  block. 
Along  the  Pinto  fault,  which  defines  these  mountains  on  the  west,  the  lavas 
closely  follow  the  fault,  except  for  the  short  distance,  already  mentioned, 
northeast  of  Dome  Mountain.  Richmond  Mountain  encircles  the  block  on 
the  north,  followed  on  the  northeast  by  the  broad  basaltic  flows  of  Basalt 
Peak  and  the  Strahlenberg,  which  lie  between  County  Peak  and  the  Dia- 
mond Range.  To  the  east  every  indication  points  to  the  occurrence  of  lavas 
beneath  the  alluvial  deposits  of  Newark  Valley,  while  the  Rescue  Canyon 
overflows  continue  southward  to  the  open  valley,  completing  the  circuit  in 
this  direction.  Facing  Fish  Creek  Valley,  bosses  and  knolls  of  both  acid 
and  basic  lava  stretch  westward  in  sufficient  number  to  plainly  suggest  a 
continuous  outbreak  of  igneous  material  along  the  southern  base  of  the 
uplifted  block  of  Silurian  and  Devonian  limestones. 

In  the  case  of  the  depressed  Carboniferous  area  the  bordering  lines 
of  lava  following  the  Hoosac  and  Pinto  faults  are  clearly  made  out. 
Between  the  Spring  Hill  and  the  Carbon  Ridge  bodies  there  is  a  sharp 
break  in  the  Carboniferous  strata,  along  which  acidic  lavas  have  broken 
out,  crossing  obliquely  from  the  one  great  fault  to  the  other.  On  the  north 
side  of  the  Spring  Hill  body,  Richmond  Mountain  cuts  off  everything  to 
the  northeast.  The  volcanic  pumices  and  tuff  stretch  westward  under  the 
town  of  Eureka  and  terminate  finally  in  the  rhyolites  of  Purple  Hill.  To 
the  south  of  the  Carboniferous  area  pumices  and  tuffs  abut  against  the  base 
of  Carbon  Ridge,  skirting  the  foothills,  unless  concealed  beneath  recent 
deposits.  South  of  Gray  Fox  Peak  and  Carbon  Ridge  there  is  a  long  line 
of  secondary  ridges,  now  completely  covered  by  Quaternary  deposits.  It 
is  easily  seen  that  their  trend  is  wholly  out  of  accord  with  the  line  of  the 
Paleozoic  uplifts,  but  is  precisely  what  we  might  expect  to  find  if  the  lavas 


OCCUKRENCE  OF  DIKES.  247 

had  poured  out  along  the  foothills  and  encircled  the  terminal  spurs  of  the 
upturned  Paleozoic  beds.  Pumices  and  tuffs  again  come  to  the  surface 
along  the  southern  end  of  the  Pogonip  beds,  skirt  Devonian  limestone  upon 
the  south  side  of  South  Hill,  and  thence,  penetrating  the  mountains,  follow 
up  Grays  Canyon  on  the  west  side.  By  reference  to  the  atlas  sheets  it  will 
be  readily  seen  that  the  lavas  border  the  depressed  areas  of  Carboniferous 
rocks  lying  between  the  two  great  faults  in  as  forcible  a  manner  as  they  do 
in  the  case  of  the  elevated  County  Peak  and  Silverado  orographic  block. 

intrusive  Dikes.— Dikes  of  andesite,  rhyolite,  and  basalt  penetrate  the  strata 
in  a  number  of  localities,  for  the  most  part,  except  in  the  case  of  rhyolites,  in 
close  proximity  to  the  principal  lines  of  volcanic  activity.  That  they 
possess  the  same  deep-seated  origin  with  the  larger  bodies  seems  evident 
from  their  position  and  similarity  of  petrographical  characters,  their  mode 
of  occurrence  clearly  suggesting  that  they  are  merely  offshoots  from  parent 
magmas.  The  erupted  material  was  forced  upward  into  narrow  fissures 
and  fractures,  following  lines  of  least  resistance.  In  their  geographical 
distribution  they  present  some  striking  differences,  andesitic  dikes  being 
found  only  to  the  west  of  the  Pinto  fault,  and  for  the  most  part  confined 
to  Cambrian  and  Silurian  rocks  of  the  Prospect  Ridge  uplift,  whereas 
basaltic  dikes  arrange  themselves  around  the  County  Peak  and  Silverado 
Mountain  body.  Rhyolite  dikes,  while  they  may  break  out  anywhere 
along  lines  of  displacement,  offer  a  marked  geological  feature  of  Prospect 
Ridge,  the  eastern  slope  being  cut  by  a  network  of  intrusive  bodies. 
They  vary  from  thirty  feet  to  a  few  inches  in  width,  and  trend  at  all  angles, 
some  of  them  agreeing  with  the  strike  of  the  beds,  while  a  few,  notably  the 
Geddes  and  Bertrand  dike,  cross  the  strata  nearly  at  right  angles  to  the 
course  of  the  main  ridge.  The  Ruby  Hill  fault-plane  is  coincident  with  a 
narrow  fissure,  into  which  the  rhyolitic  magma  has  forced  an  entrance  for 
the  greater  part  of  its  length,  forming  the  most  persistent  dike  of  any  in 
the  region.  In  the  neighborhood  of  the  Dunderberg  and  Hamburg  mines 
numerous  outbursts  of  rhyolite  have  reached  the  surface.  Notwithstanding, 
however,  the  great  number  of  these  dikes,  none  appeal-  to  have  penetrated 
the  strata  along  the  top  of  the  main  ridge,  and  in  no  single  instance  have 
lavas  built  up  any  considerable  knob  or  hill  on  the  surface.  It  is  quite 


248  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

impossible  that  such  knobs  should  have  been  formed  and  later  have  been 
removed  by  erosion  without  leaving  some  evidence  of  overflow  along  the 
line  of  the  dikes. 

This  system  of  dikes  upon  Prospect  Ridge  presents  certain  geological 
characteristics  of  interest  bearing  upon  the  mode  of  occurrence  of  erupted 
material.  Throughout  they  show  a  great  similarity  in  mineral  composi- 
tion and  petrographical  habit,  and  when  fresh  in  every  way  resemble  the 
unaltered  rocks  along  the  Ruby  Hill  fault.  These  latter  lavas  have  been 
shown  elsewhere  to  have  been  erupted  at  the  same  time  and  under  similar 
conditions  with  the  rhyolite  of  the  Hoosac  fault.  Indeed,  the  Ruby  Hill 
fault  is  simply  a  prolongation  of  the  main  fault.  Evidences  of  alteration 
and  metamorphism  of  the  limestones  and  shales  through  which  the  erupted 
material  passed  are  by  no  means  easy  to  detect,  the  encasing  walls  showing 
scarcely  any  evidence  of  the  effects  of  heat  derived  from  ascending  lava 
currents.  These  dike  rocks  being  narrow  bodies  have  cooled  rapidly  and 
imparted  little  heat  to  the  limestones.  Mining  exploitations  have  frequently 
encountered  these  intrusive  bodies  hundreds  of  feet  below  the  surface,  but 
neither  at  the  top  nor  underground  do  they  exhibit  structural  features  in 
any  way  different  from  the  larger  bodies.  The  only  marked  feature  in 
which  these  dike  rocks  differ  from  the  extrusive  lavas  of  Pinto  and  Gray 
Fox  peaks  is  shown  by  the  absence  of  flow  structure  due  entirely  to  their 
manner  of  occurrence,  and  in  no  way  dependent  upon  either  their  chem- 
ical or  mineralogical  composition.  As  regards  the  degree  of  ciystalliza- 
tion,  they  exhibit  characters  identical  throughout  and  similar  to  the  material 
erupted  at  the  surface  along  the  principal  lines  of  faulting. 

On  all  the  great  lines  of  orographic  fracture  along  which  both  acid 
and  basic  lavas  have  emanated,  the  amount  of  volcanic  material  reaching 
the  surface  has  varied  greatly  at  different  points.  In  certain  localities 
they  have  piled  up  to  such  an  extent  as  to  form  prominent  hills  and 
landmarks,  but  their  mode  of  occurrence  is  precisely  the  same  as  those 
where  the  lavas  have  only  accumulated  in  narrow  belts  along  the  fissures. 
Such  masses  as  Pinto  Peak,  Purple  Hill,  and  Gray  Fox  Peak  are  similar 
piles  of  lava,  uniform  in  character,  only  varying  in  size  according  to  the 
amount  thrown  out  at  each  locality.  In  the  same  way  Richmond  Moun- 


RELATIVE  AGE  OF  LAVAS.  249 

tain  is  a  vast  accumulation  of  pyroxene-andesite  similar  in  its  geological 
occurrence  to  the  smaller  hills  of  basalt  which  have  broken  out  at  numer- 
ous points  along  the  fractures  caused  by  the  elevation  of  the  County  Peak 
and  Silverado  block. 

• 

Mr.  Clarence  King,  in  summing  up  the  observations  of  the  geologists 
connected  with  the  Geological  Exploration  of  the  Fortieth  Parallel  upon 
the  mode  of  occurrence  of  the  rhyolites  between  the  Sierra  Nevada  and 
Wasatch ranges,  makes  the  following  concise  generalization: 

Where  a  great  mountain  block  has  been  detached  from  its  direct  connections 
and  dropped  below  the  surrounding  levels,  there  the  rhyolites  have  overflowed  it  and 
built  up  great  accumulations  of  ejecta.  Wherever  the  rhyolites,  on  the  other  hand, 
accompany  the  relatively  elevated  mountain  blocks,  they  are  present  merely  as  bor- 
dering bauds  skirtiug  the  foothills  of  the  mountain  mass.  There  are  few  instances 
in  which  hill  masses  were  riven  by  dikes  from  which  there  was  a  limited  outflow  over 
the  high  summits;  but  the  general  law  was,  that  the  great  ejections  took  place  in 
subsided  regions.1 

Nowhere  within  the  Great  Basin  does  this  description  hold  true  with 
greater  force  than  in  the  Eureka  District.  It  holds  true,  however,  for  the 
entire  hornblende-andesite  and  dacite  groups,  as  in  their  mode  of  occurrence 
they  can  not  be  separated  from  the  more  acidic  lavas.  It  holds  equally 
well  for  the  pyroxene-andesites,  since  such  broad  masses  as  make  up  Rich- 
mond Mountain  are  simply  relatively  large  accumulations  of  lavas  at  cen- 
ters of  great  dislocation  in  highly  disturbed  regions  in  every  way  similar  to 
those  of  other  lavas.  In  the  case  of  the  hornblende-andesites  and  rhyolites 
they  have  poured  over  and  nearly  submerged  a  depressed  sedimentary 
region,  whereas  the  rhyolites,  pyroxene-andesites,  and  basalts,  which  have 
broken  out  in  proximity  to  the  Silverado  and  County  Peak  region,  appear 
more  as  an  encircling  belt  to  a  relatively  elevated  country. 

Relative  age  of  Volcanic  Rocks.— In  the  Eureka  District  the  hornblende- 
andesite  and  the  closely  related  hornblende-mica-andesite  are  the  earliest 
of  the  Tertiary  lavas,  all  others  with  which  they  are  associated  being  found 
either  to  break  through  or  overlie  them.  Hornblende-andesite,  wherever  it 
occurs  in  the  district,  is  a  crystalline  rock  and  forms  a  central  body,  which, 
by  insensible  transitions,  passes  into  a  rock  with  a  more  and  more  glassy 

'U.  S.  Geol.  Explor.  40th  Par.,  1878,  vol.   I,  Systematic  Geology,  p.  6W. 


250  GEOLOGY  OP  THE  EUREKA  DISTRICT. 

base  until  it  becomes  a  characteristic  audesitic-pearlite.  As  the  andesites 
and  pearlites  become  more  and  more  acidic  the  rock  gradually  passes  over 
into  dacite,  the  eruptions  of  which  usually  occur  in  obscure  hills  and  low 
ridges,  and  although  covering  comparatively  restricted  areas  are  clearly 
seen  to  overlie  the  hornblende-mica-andesite  in  all  the  local  centers  of 
eruption  wherever  the  two  rocks  are  observed  together.  In  the  neighbor- 
hood of  South  Hill,  where  the  largest  exposures  of  dacite  have  been 
observed,  they  rest  superimposed  against  the  andesite,  and  at  Dry  Lake, 
where,  however,  only  a  small  body  of  dacite  is  known,  it  is  evident  that 
a  similar  sequence  of  flow  was  maintained. 

In  low  hills  near  the  entrance  to  Sierra  Canyon  northeast  of  South 
Hill  instances  may  be  seen  of  finely  banded  rhyolite  lying  in  direct  super- 
position upon  good  exposures  of  dacite.  This  dacite,  though  a  moderately 
compact  rock,  possesses  in  places  a  pumiceous  texture  and  in  a  marked 
degree  strongly  resembles  many  forms  of  rhyolite,  but  especially  the  variety 
with  which  it  is  here  associated.  Both  rocks  are  highly  acidic,  but  the 
dacite  is  richer  of  the  two  rocks  in  mineral  secretions  and  is  character- 
ized by  a  great  abundance  of  laminae  of  biotite.  In  the  few  areas  where 
both  rocks  occur  together  in  such  a  way  that  their  relations  can  be  made 
out,  the  rhyolite  has  been  the  last  to  reach  the  surface. 

The  district  affords  abundant  and  frequent  evidence  of  the  relative 
geological  position  of  andesite  to  rhyolite.  Not  only  is  this  shown  by 
the  relationship  between  the  rhyolites  and  dacites,  but  over  much  more 
extended  areas  the  rhyolite  encircles  and  overlies  the  andesite,  filling  in  and 
smoothing  out  the  accidented  surface  of  the  older  rock,  which  in  turn  may 
occasionally  be  seen  in  isolated  exposures  rising  above  a  broad  expanse  of 
superimposed  rhyolite.  Further  and  conclusive  evidence  is  found  in  the  fre- 
quent dikes  of  rhyolite  penetrating  the  hornblende-mica-andesite  in  several 
places  adjoining  the  Hoosac  fissure.  The  rhyolitic  pumices,  tuffs  and  allied 
rocks  appear  in  many  instances  to  have  preceded  the  more  highly  crystalline 
compact  rhyolites  represented  by  the  typical  Rescue  Canyon  and  Pinto 
Peak  rocks. 

While  it  is  by  no  means  evident  that  all  the  overflows  of  pumice 
broke  out  before  the  denser  rock,  yet  there  is  ample  proof  that  long  and 


RELATIVE  A(JE  OE  LAVAS.  251 

continuous  bodies  spread  out  over  wide  areas  of  country,  especially  along 
the  line  of  the  Pinto  fault,  before  the  great  bodies  of  the  latter  were  forced 
to  the  surface.  Rhyolites  occur  breaking  through  the  pumices,  overflowing 
and  occasionally  concealing  them  from  view,  except  where  the  softer  rock 
is  exposed  by  deep  cuts  along  drainage  channels.  In  some  instances  the 
pumices  lie  superimposed  upon  denser  rock,  evidently  of  later  age.  It 
seems  most  probable  that  throughout  the  duration  of  rhyolitic  eruptions 
conditions  were  at  all  times  more  or  less  favorable  for  the  pouring  out  of 
pumices  and  tuffs,  and  that  outbursts  of  similar  material  began  and  closed 
the  rhyolite  period.  The  conditions  governing  the  physical  characteristics 
of  the  erupted  material  seem  in  a  great  measure  to  have  been  dependent 
upon  their  relations  to  certain  local  centers  of  volcanic  activity. 

Along  the  .Pinto  fault,  wherever  the  acidic  lavas  have  piled  up, 
pumices  occur  as  the  prevailing  rock,  and  the  same  holds  true  along  the 
lines  of  displacement  bordering  the  elevated  mountain  masses.  Normal 
crystalline  rhyolite,  on  the  other  hand,  characterizes  the  Hoosac  fault  and 
breaks  out  wherever  these  lavas  penetrate  into  the  interior  of  the  mountains 
along  fissures  and  lines  of  least  resistance.  They  frequently  reach  the 
surface  in  small  isolated  bodies  in  the  most  distant  and  unlooked-for  places- 
The  Rescue  fault  is  an  instance  of  rhyolite  penetrating  into  the  very 
center  of  the  mountains,  and  the  pumices  and  tuffs  on  the  south  side  of  the 
Silverado  Mountains  offer  a  fine  example  of  the  pouring  out  of  the  latter 
along  the  outer  edge  of  an  uplifted  orographic  block.  Following  lines  of 
least  resistance  they  connect  the  rhyolites  of  the  Rescue  with  those  of  the 
Pinto  fault. 

When  it  comes  to  determining  the  geological  relations  of  pyroxene- 
andesite  to  hornblende-andesite  and  allied  lavas,  no  direct  superposition  can 
be  found,  nor  are  there  any  instances  of  dikes  of  one  rock  breaking  through 
an  earlier  body  of  the  older  rock.  The  main  bodies  of  hornbleiide-andesite 
and  pyroxene-andesite  are,  as  regards  geological  position  and  geographical 
distribution,  quite  distinct. 

Absence  of  direct  evidence  as  to  the  relative  age  of  the  two  large 
groups  of  andesite  may  be  explained  satisfactorily  by  the  fact  that  only  one 
body  of  pyroxene-andesite  occurs  in  the  district  and  this  one,  although 


252  G-EOLOGY  OF  THE  EUREKA  DISTRICT. 

covering  an  extensive  area  and  of  great  thickness,  has  no  outlying 
exposures.  The  Cliff  Hills  which  lie  beyond  the  limits  of  the  Eureka 
Mountains,  present  a  grand  exposure  of  pyroxene-andesite,  but  as  they  stand 
alone  afford  no  evidence  as  to  the  relations  of  the  different  lava  flows  to 
each  other.  Along  the  base  of  the  escarpment,  which  forms  the  south  side 
of  Richmond  Mountain,  occurs  a  contact  over  a  mile  in  length,  between 
pyroxene-andesite  and  rhyolitic  pumices,  yet  nowhere  along  this  line  has 
the  sequence  of  eruption  been  definitely  determined  by  actual  contact. 
Evidence  fails  to  show  whether  the  pyroxene-andesite  broke  through  the 
pumices,  which,  on  account  of  their  friable  nature  have  suffered  more  or  less 
erosion,  or  whether  the  latter  banked  up  against  a  preexisting  wall  of  the 
former.  For  the  greater  part  of  the  distance  the  junction  of  the  two  rocks 
is  completely  obscured  by  both  large  and  small  blocks  of.  andesite,  which 
have  fallen  from  the  cliff  above,  and  wherever  these  are  wanting  the  con- 
tact is  hidden  by  fine  friable  pumice  and  ash,  which  has  accumulated  in 
considerable  thickness  along  the  base  of  the  escarpment,  piled  up  by  the 
prevailing  westerly  winds.  Although  no  actual  superposition  is  seen,  all 
indirect  evidences  point  so  strongly  to  the  true  order  of  succession  that  the 
fact  seems  well  established  that  the  pyroxene-andesite  followed  the  rhyo- 
lite. 

Between  the  pyroxene-andesites  and  basalts  there  exists  the  closest 
possible  relationship,  so  much  so  that  it  is  by  no  means  an  easy  matter  to 
establish  a  sharp  line  between  them,  either  in  mineral  composition  or  field 
occurrence.  Unlike  pyroxene-andesite,  however,-  the  outbursts  of  basalt 
present  a  considerable  diversity  in  their  mode  of  occurrence  and  distribu- 
tion, forming  broad  table-like  masses  and  numerous  small  extrusions  in  dikes 
and  rounded  knolls.  Although  the  two  rocks  are  closely  related  by  transi- 
tion products,  extreme  typical  forms  may  easily  be  distinguished  from  each 
other  by  both  geological  and  petrographical  features  of  rock  masses,  and 
as  to  their  order  of  succession  there  exists,  fortunately,  abundant  proof  to 
show  that  the  pyroxene-andesite  preceded  the  basalt.  Evidences  of  their 
relative  age  may  be  seen  on  the  summit  of  Richmond  Mountain,  where 
several  dikes  of  dense  glassy  basalt  cut  the  andesite  in  sharply  defined  lines 


SEQUENCE  OF  LAVAS.  253 

of  contact,  and  at  several  localities  near  the  outer  edge  of  the  andesitic  body, 
notably  just  east  of  the  town  of  Eureka. 

Now,  the  relationship  as  regards  age  between  the  basalts  and  rhyolites 
is  placed  beyond  all  question,  numerous  dikes  of  the  former  cutting  the 
latter  both  along  the  Pinto  fault  and  in  the  pumice  basin  southwest  of  Rich- 
mond Mountain.  Hornitos  Cone,  about  400  feet  in  height,  an  isolated  hill 
rising  abruptly  out  of  the  basin,  is  an  excellent  instance  of  the  cutting  of 
rhyolite  by  basalt  dikes.  The  cone  is  composed  of  light  colored  pumices, 
broken  through  and  ribbed  on  all  sides  by  black  basaltic  dikes,  which  have 
altered  the  siliceous  rocks  all  along  the  lines  of  contact.  Crater  Cone,  on 
the  east  side  of  Richmond  Mountain,  affords  an  equally  good  example  of 
the  relative  position  of  the  two  rocks,  the  basaltic  lavas  which  here  form 
the  Cone  flowing  for  long  distances  over  the  earlier  pumiceous  beds.  Mag- 
pie Hill,  near  the  entrance  to  Rescue  Canyon,  affords  still  another  equally 
as  good  an  illustration  of  the  relative  position  of  the  two  rocks. 

If  the  pyroxene-andesite  overflows  preceded  the  rhyolite  it  would 
hardly  have  been  possible  under  the  conditions  of  eruption  for  them  not  to 
have  broken  out  along  some  of  the  hornblende-andesite  centers  before  the 
appearance  of  the  rhyolites.  Again,  if  the  rhyolites  followed  the  pyroxene- 
andesite  there  should  be  found  some  field  evidences  of  such  eruptions 
between  the  pyroxene-andesite  and  basalt,  whereas,  on  the  contrary,  there 
exists  not  the  slightest  evidence  of  an  overflow  of  acidic  lava  intervening 
between  the  closely  related  basic  lavas.  It  has  already  been  pointed  out 
that  the  acidic  lavas  hold  the  same  close  relationship  to  each  other. 

Field  observations  clearly  show  that  the  order  of  succession  of  these 
natural  groups  into  which  the  lavas  have  been  divided  was  as  follows : 
First,  that  the  hornblende-andesite  was  the  earliest  of  all  the  erupted 
material ;  second,  that  the  homblende-mica-andesite  followed  the  hornblende- 
andesite;  third,  that  the  dacite  followed  the  hornblende-mi ca-andesite; 
fourth,  that  the  rhyolite  closely  followed  the  dacite;  fifth,  that  the  pyroxene- 
andesite  succeeded  the  rhyolite;  sixth,  that  the  basalt  was  the  most  recent 
of  all  volcanic  products. 

TWO  Magmas  of  Eruption.— A  study  in  the  field  of  the  geological  distribu- 
tion and  mode  of  occurrence  of  the  igneous  rocks,  shows  that  they  all  belong 


254  G  KG  LOGY  OF  THE  EUREKA  DISTRICT. 

to  one  or  the  other  of  two  well  defined  groups,  in  each  of  which  the  lavas, 
although  possessing  a  wide  range  in  chemical  composition,  are  so  intimately 
related  and  so  interdependent  as  to  suggest  that  they  must  necessarily  have 
been  derived  from  some  common  source.  In  other  words,  all  lavas  at  Eureka 
may  be  divided  into  two  sharply  contrasted  groups,  the  one  acid,  and  the 
other  basic. 

A  microscopical  examination  in  the  laboratory  of  a  large  amount 
of  material  collected  in  the  field  in  the  opinion  of  the  writer 
lends  support  to  this  view  of  two  magmas.  It  is  brought  about  by 
a  study  of  the  gradual  transition  in  mineral  composition  and  by  cer- 
tain peculiarities  of  structure  and  crystallization  characteristic  of  each 
magma.  The  acid  magma  was  the  earlier  in  age,  the  eruptions  begin- 
ning with  hornblende-andesite  and  closing  with  the  extreme  acidic  forms 
of  rhyolite.  In  general  the  lavas  of  this  acid  series  are  light  in  color, 
the  microcrystalline  groundmass  being  composed  for  the  most  part  of  an 
aggregation  of  feldspar  and  quartz  grains  without  the  accompaniment  of 
ferro-magnesian  silicates.  The  hornblendes  play  no  part  in  the  composi- 
tion of  the  groundmass,  being  present  as  porphyritic  secretions,  whereas  the 
pyroxenes,  in  the  few  instances  where  they  have  been  recognized,  at  the 
basic  end  of  the  series  do  not  occur  as  porphyritic  minerals,  but  only  in 
minute  microlitic  forms  developed  in  the  groundmass.  The  glass  is  always 
highly  acidic. 

Sharply  contrasted  with  these  acidic  lavas  the  basic  lavas  are  char- 
acterized by  a  predominance  of  the  pyroxenic  minerals,  the  prevalence  of 
lime-soda  feldspars  and  the  structural  features  of  a  groundmass  peculiar  either 
to  pyroxene-andesite  or  to  basalt.  The  basic  magma  came  in  with  pyroxene- 
andesite  and  closed  with  numerous  outflows  of  basalts.  The  two  magmas 
so  sharply  defined  by  mineralogical  and  structural  distinctions  may  be 
designated  respectively  as  the  feldspathic  and  pyroxenic  magmas.  A  dis- 
cussion as  to  their  nature  will  bring  out  still  more  clearly  their  diagnostic 
points  of  difference  and  the  importance  of  this  division  in  its  bearing  upon 
the  origin  of  the  sequence  of  lavas.  Further  on  in  this  chapter  it  will  be 
maintained  that  both  these  magmas  are  simply  differentiated  products  of 
an  earlier  homogeneous  molten  mass. 


FELDSPATHIC  MAGMA.  255 

Feidspathic  Magma.— Up  to  this  point  the  composition  of  the  rocks  has 
been  but  little  considered  except  as  regards  the  mineral  constituents  of  inde- 
pendent lava  flows ;  it  is  necessary  now,  however,  to  look  at  them  from  the 
standpoint  of  a  series  of  successive  eruptions  in  order  to  understand  their 
interdependence  and  geological  relations.  Normal  hornblende-andesite,  the 
earliest  and  most  basic  portion  of  the  feldspathic  magma,  passes  over  with- 
out any  recognizable  physical  break  into  hornblende-mica-andesite  by  the 
coming  in  of  hexagonal  plates  of  biotite  which  gradually  increase  in  amount 
until  they  become  the  most  prominent  of  the  ferro-magnesian  minerals  and 
at  the  same  time  by  insensible  gradations  the  hornblendes  decrease.  Grad- 
ually the  lava  grows  more  and  more  acidic  and  quartz  grains  are  developed 
in  the  groundmass,  but  at  first  not  in  sufficient  force  to  be  regarded  as  an 
essential  constituent.  The  presence  or  absence  of  quartz  is  also  governed 
in  great  measure  by  the  degree  of  crystallization  of  the  magma,  a  highly 
crystalline  structure  carrying  more  individual  secretions  than  one  where 
silica  is  largely  absorbed  in  glass.  With  the  increase  of  quartz  the  horn- 
blende continues  to  diminish  and  the  rock  passes  over  into  dacite,  the  biotite 
apparently  holding  its  position  with  an  occasional  hornblende. 

In  dacite,  quartz  has  become  an  essential  mineral.  With  a  still  larger 
increase  of  the  silica  percentage  orthoclase  appears  in  broad  and  well  devel- 
oped crystals.  Hornblende  disappears  entirely  and  in  the  normal  varieties 
of  rhyolite  the  biotite  is  rarely  seen  and  then  only  as  an  accessory  min- 
eral ;  the  feiTO-magnesian  minerals  are  wanting.  At  the  basic  end  of  the 
feldspathic  series  of  lavas,  labradorite  and  anorthite  have  been  determined 
by  their  optical  pi-operties,  but  the  predominating  feldspars  are  apparently 
oligoclase.  By  insensible  gradation  the  lime-soda  feldspars  pass  away. 
Orthoclase,  in  most  of  the  basic  rocks,  is  entirely  wanting,  making  its 
appearance  by  degrees  until  at  the  acid  end  of  the  series  it  occurs  as  the 
prevailing  feldspar,  although  some  species  of  plagioclase  is  nearly  always 
present.  At  one  end  of  this  series  of  eruptive  material  the  essential  min- 
erals are  hornblende  and  one  or  more  species  of  lime-soda  feldspars;  at  the 
other,  quartz  and  orthoclase. 

Pyroxenic  Magma.— The  basic  or  pyroxene  lavas  began  by  the  pouring  out 
of  large  masses  of  the  Richmond  Mountain  pyroxene-andesite.  Successive 


256  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

changes  in  the  mineral  and  chemical  composition  of  this  magma  are  by  no 
means  as  easy  to  follow  through  the  different  flows  as  in  the  sequence  of 
outbursts  of  the  acidic  products.  Nevertheless,  investigation  shows  as  com- 
plete a  range  in  composition  of  the  erupted  material,  even  where  it  is 
impossible  to  determine  the  relative  age  of  the  flows  accompanying  such 
changes.  In  some  instances  in  the  more  crystalline  acidic  varieties  it  has 
been  pointed  out  that  both  hornblende  and  mica  occur  as  porphyritic  secre- 
tions, although  as  accessory  constituents,  hypersthene  being  the  predom- 
inant mineral.  In  lavas  slightly  more  basic  the  former  minerals  are  want- 
ing; hypersthene  still  plays  the  part  of  the  prevailing  ferro-magnesian 
silicate,  accompanied  by  relatively  small  amounts  of  augite,  while  in  rocks 
still  more  basic  augite  is  recognized  as  the  predominant  pyroxenic  mineral, 
accompanied  by  an  increasing  development  of  magnetite.  By  insensible 
gradations  a  series  of  hand  specimens  and  rock  sections  show  that  so  far  as 
mineral  constituents  are  concerned  the  pyroxene-andesites  pass  over  into 
basalts.  While  the  rock  masses  of  both  lavas  may  be  readily  distinguished 
in  the  field  by  marked  differences  in  physical  aspect,  it  is  by  no  means 
easy  on  a  superficial  examination  to  refer  correctly  from  hand  specimens 
certain  varieties  which  approach  each  other  in  structure  and  composition. 
Mineralogically  no  sharp  distinction  can  be  drawn  between  intermediate 
varieties,  but  careful  investigation  of  the  Eureka  rocks  brings  out  certain 
differences  which  not  only  hold  good  for  this  region,  but  probably  for  other 
areas  in- the  Great  Basin.  While  observation,  as  already  mentioned,  offers 
abundant  evidence  as  to  the  position  of  the  pyroxene-andesite  to  the  basalts 
and  divides  these  closely  connected  rocks  upon  geological  grounds,  based 
upon  their  relative  age,  the  microscope  in  a  marked  manner  corroborates 
the  distinctions  made  in  the  field.  Mr.  Iddings,  who  has  submitted  a  large 
number  of  thin  sections  of  both  pyroxene-andesite  and  basalt  to  micro- 
scopical investigation,  is  able  to  substantiate  by  structural  peculiarities  of 
the  groundmass  the  geological  divisions  observed.  He  finds  that  all  those 
rocks  which  have  been  classed  as  pyroxene-andesite  possess  their  own  micro- 
structure,  characterized  by  the  felt-like  structure  of  the  groundmass  which 
has  been  so  frequently  noticed  elsewhere.  The  typical  basalts  present  in 


OLIVINE  IN  HAS  ALT.  257 

their  structure  a  uniform  grotmdmaaa  made  up  of  coarse-grained  aggrega- 
tions of  feldspar  and  augite,  imbedded  in  a  globulitic  glass  base. 

Nearly  all  the  rocks  of  intermediate  mineral  composition  possess  the 
basaltic  habit.  Hypersthene  is  wanting  in  the  normal  basalts.  Augite  and 
magnetite,  although  essential  minerals  in  the  composition  of  both  rocks,  occur 
much  more  abundantly  in  basalts.  With  one  exception  the  microscope  has 
failed  to  detect  olivine  in  any  thin  section  of  the  lavas  classed  as  pyroxeue- 
audesite,  the  exception,  however,  furnishing  quite  a  remarkable  rock,  and 
one  that  might  with  some  reason  be  placed  among  the  basalts.  It  occurs 
in  an  obscure  exposure  or  knoll  in  Fish  Creek  Valley  just  west  of  Cliff 
Hills,  and  from  its  association,  and  still  more  from  the  fact  that  its  ground- 
mass  structure  bears  the  closest  relation  to  adjoining  rocks,  it  has  been 
referred  to  the  pyroxene-andesites.  Although  olivine  is  absent  from  the 
pyroxene-andesites  of  the  district,  it  will  not  serve,  as  has  been  suggested, 
as  a  mineralogical  distinction  to  separate  the  two  natural  groups,  inasmuch 
as  over  large  basaltic  areas  it  is  wholly  wanting.  Moreover,  within  limited 
areas,  and  apparently  in  the  same  flow,  it  may  be  present  at  one  point  and 
wanting  in  another,  occurring  so  irregularly  disseminated  through  the  rock 
that  any  attempt  to  separate  the  basalts  themselves  into  two  divisions  on  a 
basis  of  olivine  seems  futile.  In  an  abstract  of  the  geology  of  the  Eureka 
District  published  in  1883  this  relationship  between  the  olivine  and  basalt 
was  clearly  pointed  out.1  Since  then  it  has  been  shown  that  olivine  is 
absent  in  numerous  basaltic  lavas  of  the  Great  Basin.2  Mr.  George  F. 
Becker3  has  recently  arrived  at  the  conclusion  that  olivine  can  not  be  used 
as  a  basis  of  division  for  the  basalts  along  the  sierra  of  California. 

characteristic  Basalts.— It  is  well  to  mention  two  other  marked  peculi- 
arities of  these  basalts — one,  the  very  varying  amount  of  silica  which 
they  carry;  the  other,  the  very  high  percentage  of  silica  contained 
in  the  rock  as  compared  with  the  most  basaltic  flows  elsewhere.  In 
their  chemical  composition  nearly  all  these  rocks  possess  far  more  silica 

1  Third  annual  report  of  the  Director  of  the  U.  S.  Geological  Survey,  1881-'82. 

a  Arnold  Hague  and  Jos.  P.  Iddiugs:  Notes  on  the  volcanic  rocks  of  the  Great  Basin.     Am.  Jour. 
Sci.,  June,  1884,  p.  157. 

"Geology  of  the  quicksilver  deposits  of  the  Pacific  Slope.     Monograph  XIII,  I'.  S.  Geol.  Survey. 
1888,  p.  157. 

MON  XX 17 


258 


GEOLOGY  OF  THE  EUKEKA  D1STKICT. 


than  is  ordinarily  supposed  to  occur  in  normal  basalt,  the  amount  reaching 
as  high  as  the  percentage  found  in  many  andesitic  rocks,  and  in  some 
instances  equaling  the  amount  in  the  pyroxene-audesite  of  Richmond 
Mountain. 

oiivine  in  Basalts.— In  order  to  determine  the  amount  of  silica  present  in 
these  rocks  and  its  relationship  to  oiivine,  a  number  of  chemical  analyses 
were  made  from  specimens  which  field  observation  and  a  study  of  thin 
sections  had  shown  to  belong  to  basalt.  The  subjoined  table  gives  the 
result  of  ten  such  chemical  examinations,  arranged  in  order  according  to  the 
silica  percentage  obtained.  The  presence  or  absence  of  oiivine  in  the  thin 
sections  of  the  same  rocks,  as  determined  by  the  microscope,  is  also  given 
in  the  table. 


Number. 

Silica. 

Oiivine. 

1 

49.23 

Rich  in  macroscopic  secretions. 

2 

51.86 

Rich  in  microscopic  secretions. 

3 

57.42 

Abundant  in  microscopic  secretions. 

4 

58.06 

Easily  recognized  under  the  microscope. 

5 

58.26 

Only  a  trace. 

6 

58.60 

None  detected. 

7 

58.64 

Detected  under  the  microscope. 

8 

59.51 

None  detected. 

9 

59.64 

None  detected. 

10 

60.11 

None  detected. 

No.  1.  South  of  Alhambra  Hills. — This  rock  occurs  as  a  low  hill  rising  out  of  the  Quaternary 
plain,  and  isolated  from  all  other  volcanic  outbursts.  It  is  a  highly  crystalline  rock. 

_Yo.  2.  Dike  northeast  of  summit  of  Richmond  Mountain. — An  intrusive  body  penetrating  the  pyr- 
oxeiie-andcsiti-. 

No.  3.  East  of  Basalt  Peak. — A  vesicular  black  basalt. 

No.  4.  liasalt  Cone. — A  compact  dark  rock  characteristic  of  a  large  area  of  country. 

No.  5.  Basalt  Peak. — A  compact  rock  passing  into  vesicular  varieties. 

A'o.  6.  West  base  of  Richmond  Mountain,  near  the  town  of  Eureka. — A  grayish  red  vesicular  rock 
lying  between  pyroxeuc-andcsite  and  earlier  rhyolitic  tuffs. 

No.  7.  West  of  Basalt  Peak. — It  occurs  on  the  broad  saddle  just  west  of  the  peak,  not  far  from 
the  groat  body  of  Devonian  limestone,  and  is  a  characteristic  rock  rich  in  glass  base,  black  in  color, 
mottled  with  gray. 

No.  S.  West  bate  of  Richmond  Mountain,  nut  far  from  the  town  of  Eureka. — In  its  geological  rela- 
tions it  is  quite  similar  to  No.  6.  It  is  found  breaking  through  rhyolitic  tuffs  and  is  a  compact  dark 
rock  with  a  characteristic  basaltic  habitus. 

No.  0.  A  dike  from  the  summit  of  Richmond  Mountain. — Under  the  microscope  the  rock  resembles 
No.  2,  which  occurs  not  far  distant,  penetrating  the  same  body  of  andesite  under  precisely  similar 
geological  conditions.  This  rock,  however,  is  much  richer  in  glass  and  correspondingly  richer  in 
silica.  It  is  black  in  color,  without  macroscopic  secretions,  and  has  a  decidedly  conchoidal  fracture. 


OUYINK   IN    P.ASALT.  1>,V.) 

No.  10.   Went  of  Tiill   llnad,  went  of  Dome  Mountain. — It  ooeun  ac  one  of  the  largest  extniitlonB  of 

basalt  along  the,  Piuto  fault.  The  bcoad  mass  lies  iu  contact  with  hornblende -andesite,  and  Hows 
from  the  same  body  arc  tteeii  to  directly  overlie  rhyolitie  tuffs.  It  in  exceedingly  rich  iii  glass,  "nd  so 
mottled  as  to  present  a  gray  color.  Although  the  highest  on  the  list  in  the  percentage  of  silica,  if 
possesses  a  Strongly  marked  basaltic  habitus,  quite  as  characteristic  under  the  microscope  as  in  the 
hand-specimen. 

It  will  be  seen,  with  the  exception  of  numbers  one  and  two,  that  the 
silica  percentage  in  all  the  rocks  is  higher  than  is  usually  found  in  basalts  ; 
they  show  between  the  two  extremes  on  the  list  a  variation  in  silica  of  10'88 
per  cent. 

Although  olivine  is  not  an  essential  constituent  in  the  basalts,  the  above 
table  shows  how  close  a  relationship  exists  between  the  olivine  bearing 
and  olivine  free  varieties,  and  a  study  of  the  localities  and  their  mode  of 
occurrence  demonstrates  how  futile  an}'  attempt  would  be  to  try  to  sep- 
arate them  on  the  presence  or  absence  of  this  mineral.  In  the  hill  south 
of  Alhambra  Hills,  the  silica  is  low,  while  the  olivine  is  present  in  com- 
paratively large  secretions.  In  the  dike  from  the  summit  of  Richmond 
Mountain,  the  second  in  the  table,  there  is  an  increase  in  the  amount  of 
silica  of  over  2'5  per  cent,  with  a  large  falling  off  in  olivine.  From  the 
rocks  with  58  to  59  per  cent  of  silica,  there  is  only  a  small  and  varying 
quantity  of  olivine,  while  in  the  three  specimens  which  gave  over  59  per 
cent  of  silica  the  microscope  failed  to  detect  its  presence. 

Sufficient  facts  have  been  adduced  to  indicate  how  intricately  the 
entire  series  of  pyroxenic  rocks  are  related  to  each  other  throughout  a  wide 
range  in  their  composition.  Throughout  this  entire  group  of  extravasated 
lavas  the  essential  minerals  remain  the  same,  the  differences  consisting 
for  the  most  part  in  their  relative  proportions  and  the  accompanying 
modifications  of  groundmass  structure.  This  holds  true  in  a  still  more 
striking  manner  if  we  exclude  the  extreme  acidic  end  of  the  series  where 
the  hornblende  and  mica  play  the  part  of  accessory  minerals.  Some  of 
the  basaltic  masses  determined  as  such  by  geological  position  and  structural 
peculiarities  have  been  found  in  several  instances,  usually  the  more  glassy 
varieties,  to  be  more  acidic  than  the  pyroxene-andesites,  the  two  natural 
groups  overlapping  each  other  as  regards  their  composition.  The  sudden 
changes  which  all  these  pyroxeuic  lavas  apparently  undergo  from  crystalline 
to  glassy  varieties  is  one  of  the  marked  peculiarities  of  the  Eureka  District 


260  GEOLOGY  OF  THE  EUKEKA  D1STKIOT. 

and  with  these  changes  occur  more  or  less  variation  in  both  mineral  and 
chemical  composition. 

Age  of  Pyroxene-andesites  Elsewhere.— Similar     Surface     flows      of     pyTOXeiie- 

andesites  occur  at  numerous  localities  in  the  Great  Basin,  all  the  way  from 
the  Sierra  Nevada  Range  to  the  Salt  Lake  Desert,  although  not  always  in 
as  large  bodies  as  Richmond  Mountain,  nor  always  associated  with  basalts. 
They  are  best  shown  along  the  TruckeeCanyonin  the  Virginia  Range,  and 
in  the  Augusta,  Cortez,  and  Wahweah  ranges.  In  the  Wahweah  Range  lavas 
which  were  considered  by  Prof.  Zirkel  as  augite-trachytes  can  not  be  dis- 
tinguished from  the  Richmond  Mountain  rock  in  any  of  their  petrograph- 
ies] features.  In  the  opinion  of  the  writer  many  bodies  of  lava  which 
formerly  were  classed  as  augite-trachytes,  augite-andesites,  and  basalts, 
properly  belong-  to  this  group  of  pyroxene-andesites,  and  in  some  instances 
rocks  which  had  been  determined  as  rhyolite  from  the  fact  that  they  were 
supposed  to  cany  large  amounts  of  sanidine  have  within  recent  years  been 
shown  to  belong  to  this  same  natural  group. 

The  Eureka  District  offers  no  positive  direct  evidence  from  super- 
position of  the  relative  age  of  the  hornblende-andesite  and  pyroxene- 
andesite,  but  this  apparent  break  in  the  chain  of  evidence  is  more 
than  made  good  elsewhere,  inasmuch  as  pyroxene-andesites  of  the 
Richmond  Mountain  type  have  been  observed  breaking  through 
hornblende-andesites  not  unlike  those  found  along  the  line  of  the 
Hoosac  fault.  Similar  volcanic  rocks,  as  regards  porphyritic  secre- 
tions and  groundmass  structure,  have  been  described  by  Mr.  S.  F. 
Emmous'  as  cutting  through  and  overlying  the  homblende-andesites  in  the 
Augusta  Mountains,  both  in  the  region  of  Crescent  Peak  and  Antimony 
Canyon.  In  the  Truckee  Canyon,  rocks  which  have  been  called  augite- 
andesites  can  not  be  distinguished  from  those  of  Richmond  Mountain. 
They  were  observed  by  the  geologists  of  the  Fortieth  Parallel  Exploration 
to  break  through  sanidine-trachytes  (hornblende-mica-andesites)  and  were 
regarded  by  them  at  that  time  as  an  exception  to  the  natural  order  of 
succession,  all  andesites  being  supposed  to  be  older  than  the  so-called 
trachytes.  Along  the  walls  of  the  same  deep  gorge  and  in  its  lateral 

•U.S.  Geol.  Explor.  40th  Par.,  vol.  11,  p.  654. 


ANDESITE  LATER  THAN   RHYOLITE.  261 

branches  pyroxene-andesite  is  exposed  overlying  rhyolite1  and  for  the  same 
reason  was  regarded  as  an  anomalous  occurrence,  whereas  it  is  now  evident 
that  it  belongs  more  properly  to  that  group  of  pyroxene-andesite  which 
is  found  associated  with  and  passing  over  into  basalt.  Inasmuch  as 
it  distinctly  overlies  the  adjoining  rhyolite  it  was  designated  on  the 
geological  maps  of  the  Fortieth  Parallel  Exploration  as  basalt,  although 
in  the  text  mention  was  made  of  its  andesitic  character.  At  Jacob's 
Promontory,  in  the  Shoshone  Range,  a  body  of  lava  which  had  been 
determined  as  rhyolite  has  also  proved  on  further  examination  to  be 
allied  to  pyroxene-andesite,  and  here,  as  at  Eureka,  it  is  found  associated 
with  basaltic  flows,  although  of  earlier  age  but  overlying  typical  rhyolite. 
Numerous  localities  might  be  mentioned  where  similar  pyroxene-andesites 
occur,  but  their  relationship  with  neighboring  rhyolites  is  obscure.  Nearly 
similar  pyroxene-andesites  occur  throughout  California,  according  to  the 
descriptions  given  by  Mr.  George  F.  Becker,'  who  has  also  identified  these 
lavas  from  the  west  side  of  the  Sierras  with  similar  andesites  in  the  neigh- 
borhood of  Steamboat  Springs,  Nevada,  which  closely  resemble  those  of 
Truckee  Canyon.  Quite  recently  Mr.  H.  W.  Turner3  has  reported  the 
occurrence  of  basic  andesite  overlying  rhyolite  at  a  number  of  localities 
along  the  western  Sierra  foothills. 

These  instances  suffice  to  show  that  this  type  of  rock  occurs  over 
widely  separated  areas,  but  it  should,  however,  as  regards  its  geological 
position,  in  110  way  be  confounded  with  an  older  body  of  pyroxene-andesite 
of  somewhat  similar  composition,  such  as  is  well  represented  in  the  Washoe 
District  on  the  slopes  of  Mount  Davidson,  in  the  Virginia  Range.  The 
latter  in  general  present  a  high  degree  of  crystallization,  carrying  more 
porphyritic  secretions  and  consequently  less  glass.  On  the  other  hand,  the 
former  present  all  those  characters  which  ordinarily  characterize  surface 
flows,  and  are  for  the  most  part  darker  in  color,  as  they  cany  fewer  well 
developed  feldspars.  The  hornblende  and  pyroxene-andesites  of  Washoe 
have  been  well  described  elsewhere  in  numerous  publications  upon  that 
much  discussed  region.  In  the  opinion  of  the  writer  the  geologists  of  the 

1  U.  S.  Geol.  Explor.  40th  Par.,  vol.  n,  p.  830. 

2Geologyof  the  quicksilver  deposits  of  the  I'arific  Slope,  Mon.  IT.  S.  Oral.  Surv.  vol.  XIII. 

'Mohawk  Lake  Beds.     Phil.  Soc.  <•{  Wash.,  Rull.  xi,  pp.  385-410. 


262  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

Fortieth  Parallel  Exploration  were  led  into  error  in  supposing  that  all  the 
rocks  classed  as  pyroxene-andesite  in  the  Great  Basin  belong  to  the  same 
time  period  and  were  identical  as  regards  their  geological  position  in  the 
order  of  succession,  whereas  there  are  two  distinct  periods,  the  earlier  of 
which  is  represented  by  the  pyroxene-andesites  of  Washoe  and  preceded 
the  hornblende-mica-andesites,  dacites,  and  rhyolites,  and  the  latter  bv  the 
pyroxene-andesites  which  followed  the  rhyolites,  as  developed  on  so  grand 
a  scale  at  Richmond  Mountain. 

Accessory  Minerals.— Disseminated  through  the  lavas  at  Eureka  four 
minerals  have  been  recognized,  which  in  all  cases  occur  simply  as  accessory 
constituents,  as  in  no  single  instance  do  they  enter  largely  into  the  compo- 
sition of  the  rocks.  These  minerals  are  apatite,  zircon,  garnet,  and  allanite. 
Apatite  and  zircon  iu  a  perfectly  unaltered  condition  have  been  determined 
in  every  type  rock  of  both  feldspathic  and  pyroxenic  magmas.  The  apa- 
tites are  much  like  those  described  in  volcanic  rocks  elsewhere,  with  well 
developed  terminations  and  a  characteristic  basal  cleavage.  Zircons  in 
both  long,  slender  prisms  and  short,  stout,  colorless  crystals  are  by  no 
means  uncommon,  and,  judging  from  their  distribution,  occur  apparently 
uninfluenced  by  the  nature  of  the  lava,  notwithstanding  their  high  specific 
gravity.  They  are  found  especially  well  developed  in  the  andesitic  pearl- 
ites,  the  crystalline  forms,  as  drawn  by  Mr.  Iddings,  having  already  been 
employed  as  illustrations  of  microscopic  zircons  in  recent  text-books. 

The  presence  of  apatite  is  indicated  by  analyses  in  the  determination 
of  phosphoric  acid,  but  the  amount  of  zirconia  present  has  not  yet  been 
estimated  in  any  of  these  lavas.  Judging  from  the  analyses,  the  phosphoric 
acid  increases  with  the  basicity  of  the  lava,  starting  in  with  only  "06  per 
cent  in  the  rhyolite  from  Rescue  Canyon  and  reaching  '29  per  cent  in  the 
basalt  from  the  summit  of  Richmond  Mountain.  The  two  silicates,  garnet 
and  allanite,  have  been  detected  only  in  the  acidic  magmas,  but  both  of 
them  have  apparently  been  developed  in  the  same  type  of  rocks.  The 
garnets,  although  minute,  may  be  easily  recognized  by  the  naked  eye, 
standing  out  as  brilliant  dark  red  crystals  in  contrast  with  the  light  colored 
pumices,  tuffs,  and  pearlites  which  cany  them.  They  occur  in  both  the 
Rescue  Canyon  and  Pinto  Peak  rhyolites.  They  are  well  developed  at 


ACCESSORY  MINERALS.  263 

Gray  Fox  and  in  the  porous  white  tuffs  soutli  of  Richmond  Mountain. 
Microscopic  individuals  of  brown  and  reddish  brown  allanite  have  been 
determined,  almost  invariably  in  an  unaltered  state,  in  andesitic  pearlite, 
Rescue  Canyon  rhyolite,  and  in  other  very  glassy  varieties  of  rhyolite. 
The  determination  of  allanite  by  its  optical  and  crystallographic  properties, 
its  separation  by  chemical  analyses,  and  its  occurrence  in  widely  separated 
localities  prove  that  the  mineral  may  claim  recognition  as  an  accessory 
constituent  in  recent  volcanic  rocks.1 

In  addition  to  the  above  minerals  it  mav  be  well  in  this  connection  to 

«/ 

mention  two  nonessential  constituents  occurring  in  the  pyroxenic  lavas — 
tridymite  and  quartz — which,  although  of  interest  from  a  petrographical 
point  of  view  have  almost  no  bearing  upon  the  ultimate  composition  of  the 
original  molten  mass.  Tridymite  is  easily  recognized  under  the  microscope 
in  the  vesicular  rocks  of  Richmond  Mountain  in  thin  tabular  crystals  lap- 
ping over  each  other  in  the  manner  so  frequently  observed  elsewhere. 
These  leaf-like  crystals  arrange  themselves  in  clusters  lining  the  cavities. 
Identical  occurrences  of  tridymite  may  be  observed  in  similar  pyroxene- 
andesites  from  other  localities  in  the  Great  Basin,  notably  in  this  type  of 
lava  in  the  Wahweah  Range  northwest  of  Richmond  Mountain. 

Quartz  as  an  accessory  constituent  has  been  recognized  in  the  basalts 
from  a  number  of  localities  and  apparently  bears  no  relation  to  the  chemi- 
cal composition,  being  quite  as  apt  to  be  developed  in  the  normal  olivine 
basalts  as  in  the  more  siliceous  flows.  It  is  as  characteristically  displayed 
in  the  basic  rock  of  Magpie  Hill  as  in  any  other,  occurring  in  isolated  irregu- 
larly shaped  grains  encircled  on  all  sides  by  minute  augite  crystals.  Under 
the  microscope  they  have  all  the  appearance  of  being  of  primary  origin. 
Similar  quartz  grains  have  been  described  by  Mr.  Iddings2  from  New 
Mexico  and  Arizona,  their  origin  being  referred  by  him  to  physical  causes 
attending  an  earlier  stage  of  the  magma.  He  regards  the  exceptional  devel- 
opment of  the  quartz  in  these  basic  rocks  as  comparable  to  the  crystalliza- 
tion of  fayalite  in  the  lithophysse  of  rhyolitic  obsidian.  Similar  quartz 
grains  in  basalts  have  been  described  by  Mr.  J.  S.  Diller,  from  the  base  of 

'Joseph  P.  Iddings  and  Whitman  Cross:    Widespread  occurrence  of  allanite  as  an  accessory 
constituent  of  many  rocks.     Am.  .lour.  Sei.,  Aug.,  1885,  vol.  xxx,  pp.  108-111. 
'Bull.  U.  S.  Oeol.  Survey,  No.  66,  18!K). 


GEOLOGY  OF  THE  EUREKA  DISTRICT. 


Lassen  Peak  in  northern  California  and  are  also  regarded  by  him  as  of 
primary  origin.' 

chemical  Composition.— During  the  progress  of  the  investigation  upon  the 
erupted  material,  analyses  were  made  of  several  of  the  more  characteristic 
rocks,  which  are  presented  here  in  tabular  form  arranged  in  the  order  of 
their  basicity. 


1 

2 

3 

4 

5 

6 

7 

8 

9 

Silica.                           

75.69 

73.91 

73.09 

67.83 

67.03 

65  13 

61  58 

56  54 

50  38 

12.26 

15.29 

14.47 

15.  02 

16.27 

15  73 

16  34 

14  75 

19  83 

2  24 

6  05 

2.93 

0.89 

2.99 

5.16 

3  97 

1  86 

6  42 

9  29 

2  00 

0  3g 

Nickel  

0.07 

1  13 

0  77 

1  13 

3  07 

3  42 

3  62 

5  13 

7  80 

10  03 

0  29 

1  19 

1  49 

9  85 

6  51 

5  36 

Soda       

3.01 

3.62 

2.77 

2.40 

2.71 

2.93 

2.69 

2.07 

2  15 

Potash         

4.74 

4.79 

5.07 

3.20 

3.60 

3.96 

3.65 

2  96 

1  76 

Lithia              

0.06 

0.07 

0.26 

0  23 

0  28 

0  29 

1.04 

1  07 

0  58 

0  68 

0  55 

0  83 

1  19 

1  11 

1  56 

2  43 

0  64 

1  37 

Total      

99.82 

100.53 

99.52 

99.38 

100  72 

100  27 

100  26 

100  76 

100  14 

1.  Coll.  No.  163.— Rhyolite  from  Rescue  Canyon.     Analysis  by  R.  W.  Million.     1883. 

2.  Coll.  No.  m.— Rhyolite  from  top  of  Pinto  Peak.     Analysis  by  Dr.  Edward  Hart,  of  Lafayette 
College.    1883. 

3.  Coll.  No.  17a. — Rhyolite  overlying  daeite  from  northeast  of  South  Hill.     Analysis  by  R.  W. 
Mahon.     1883. 

4.  Coll.  No.  35. — Hornblende-mica-andesite  from  hill  northeast  of  Hoosac  Mountain.      Analysis 
by  R.  W.  Mahon.     1883. 

5.  Coll.  No.  69. — Daeite,  small  canyon  northeast  of  South  Hill.     Analysis  by  R.  W.  Mahon. 
1883. 

6.  Coll.  No.  71. — Andesi tic-pearl ite,  south  of  Carbon  Ridge.    Analysis  by  W.  H.  Melville.    1890. 

7.  Coll.  No.  79. — Pyroxene-andesite,  Richmond  Mountain.      Analysis  by  Dr.  Thomas  M.  Drown, 
Institute  of  Technology.     1883. 

S.  Coll.  No.  284.— Basalt  from  saddle  east  of  Hasalt  Peak.     Analysis  by  Dr.  Edward  Hart.    1883. 
0.    Coll.   No,   269.— Basalt,  summit  of   Richmond   Mountain.     Analysis  by  J.  Edward  Whit- 
field.     1886. 

These  nine  analyses  of  carefully  selected  material  represent  the  com- 
position of  the  entire  mass  of  extravasated  lavas  at  Eureka  and  show  a 
range  in  their  tenure  of  silica  of  over  25  per  cent.  Lavas  from  1  to  6, 
inclusive,  belong  to  the  feldspathic  magma,  and  those  from  7  to  9, 

'Am.  Jour.  Sci.,  3d  ser.,  1887,  vol.  xxxm,  pp.  45-50. 


CHEMICAL  COMPOSITION  OF  LAVAS.  265 

inclusive,  to  the  pyroxenic  magma.  Analyses  numbered  2,  4,  7,  8,  and  9 
give  the  composition  of  typical  rocks  from  different  natural  groups  and  of 
the  most  extensive  bodies  of  rhyolite,  hornblende-mica-andesite,  pyroxene- 
andesite,  acidic  basalt,  and  normal  basalt.  Each  of  these  five  rocks  carries 
about  6  per  cent  more  silica  than  the  one  standing  next  below  it  in  the 
series. 

All  the  vast  accumulation  of  lavas  may  be  regarded  either  as  belong- 
ing to,  or  as  variations  from,  these  main  types,  or  else  as  transition  products 
between  two  closely  related  natural  groups. 

Along  the  Hoosac  fault,  where  the  most  basic  unaltered  rocks  of  the 
feldspathic  magma  are  best  developed,  solfataric  action  has  so  decomposed 
them  that  it  becomes  a  matter  of  much  difficulty  to  determine  even  approx- 
imately their  original  basicity,  as  they  all  show  more  or  less  evidence  of 
infiltration  of  siliceous  material.  The  oldest  lavas  occurring  in  any  exten- 
sive body  and  still  preserved  in  a  fresh  condition  consist  almost  wholly  of 
hornblende-mica-andesite,  represented  by  the  rock  northeast  of  Hoosac 
Mountain,  carrying,  according  to  analysis,  6  7 '83  per  cent  of  silica.  The 
fine  rhyolite  from  Pinto  Peak,  free  from  ferro-magnesian  silicates  and  rich 
in  well  developed  orthoclase,  is  typical  as  regards  chemical  composition  of 
the  acidic  end  of  the  feldspathic  magma  along  the  same  great  line  of  dis- 
placement. 

It  will  be  noticed  that  the  dacite  from  northeast  of  South  Hill  carries  '8 
per  cent  of  silica  less  than  does  the  hornblende-mica-audesite,  whereas  on 
theoretical  grounds  it  would  be  expected  to  show  an  amount  somewhat  in 
excess,  owing  to  the  presence  of  quartz  secretions.  The  rock  was  selected 
on  account  of  its  well  recognized  geological  relations  with  an  overlying 
rhyolite  body,  an  analysis  of  which,  for  comparison,  will  be  found  in  the 
table.  Normal  dacite  of  the  Great  Basin  usually  carries  about  70  per  cent 
of  silica,  whereas  this  rock  stands  as  an  intermediate  variety  between  it 
and  the  andesite.  A  study  of  the  chemical  analysis  explains  the  mineral 
composition.  The  large  amount  of  iron  and  magnesia  in  excess  of  that 
found  in  the  rhyolite  and  the  falling  away  in  the  percentage  of  potash  are 
sufficient  to  account  for  both  the  predominance  of  biotite  and  the  absence 
of  sanidine.  The  plagioclastic  nature  of  the  prevailing  feldspar  assigns  the 


266  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

rocks  to  the  andesites,  while  the  presence  of  quartz  as  an  essential  con- 
stituent .places  it  more  correctly  among  the  dacites.  For  the  erupted 
material  of  Eureka  it  stands  as  one  of  the  most  basic  rocks  of  the  feld- 
spathic  magma,  rich  in  porphyritic  quartz  secretions. 

The  most  basic  of  the  feldspathic  lavas  analyzed  is  an  andesitic  pearl- 
ite,  very  limited  in  extent,  containing  65'13  per  cent  of  silica,  the  complete 
analysis  of  which  will  be  found  in  column  6  of  the  table.  It  carries  well 
developed  feldspars,  with  some  hornblende  and  biotite,  but  is  especially 
noticeable  for  the  numerous  pyroxene  microlites  which  enter  into  the 
structure  of  the  very  glassy  groundmass.  The  rock,  although  belonging 
to  the  acidic  lavas,  is  allied  to  the  basic  magma  by  the  coming  in  of  these 
microlites  of  pyroxene,  which  more  or  less  modify  the  nature  of  the  glassy 
groundmass  and  relate  it  in  structural  habit  to  the  rocks  of  Richmond 
Mountain.  It  is  doubtful  if  any  fresh  rock  of  the  feldspathic  magma  would 
fall  much  below  65  per  cent  in  silica.  An  analysis  of  a  typical  rock  from 
Richmond  Mountain,  given  in  column  7  of  the  table,  yielded  6T58  per  cent 
of  silica.  The  most  acidic  rocks  derived  from  the  pyroxenic  magma,  as 
shown  by  a  series  of  silica  determinations  in  partial  analyses,  is  62'41  per 
cent.  As  these  analyses  are  only  partial,  they  are  not  published.  They 
show  variations  from  49  to  62  per  cent  of  silica,  with  a  gradual  falling  off 
in  soda  and  potash  as  the  rocks  develop  more  and  more  magnetite  and 
olivine.  The  most  basic  basalt  examined  yielded  about  49  per  cent 
of  silica. 

By  reference  to  the  table  of  complete  analyses  it  will  be  seen  that  the 
lime,  magnesia,  and  oxides  of  iron  increase  from  the  acidic  to  the  basic  end 
of  the  series.  Of  these  bases,  lime  is  the  most  regular  in  its  behavior  and 
presents  the  widest  range,  starting  with  less  than  1  per  cent  in  the  rhyolite 
of  Pinto  Peak  and  reaching  over  ]  0  per  cent  in  the  dike  of  intrusive  basalt 
which  cuts  the  pyroxene-andesite  near  the  summit  of  Richmond  Mountain. 
It  should  be  borne  in  mind  that  the  Pinto  Peak  rock  carries  no  ferro- 
magnesian  minerals  and  the  feldspars  are  for  the  most  part  sanidine.  Mag- 
nesia stands  second  in  this  uniform  increase,  but  is  wholly  wanting  in  the 
rhyolites,  coining  in  with  the  first  appearance  of  the  ferro-magnesian- 
silicates  and  increasing  rapidly  with  the  development  of  pyroxene  and 


COMMON   SOURCE   OF  LAVAS.  267 

olivine.  In  general  both  alkalies  :nay  be  said  to  decrease  from  the  acidic 
toward  the  basic  end,  and,  except  in  the  more  basic  basalt,  the  potash 
exceeds  the  soda  in  amount. 

There  is  a  much  gi  eater  range  throughout  the  entire  series  of  lavas  in 
the  percentage  of  potash  than  in  that  of  soda,  the  former  showing  a  varia- 
tion of  over  3'25  and  the  latter  of  only  1/50  per  cent.  The  greatest  inter- 
ruption in  the  regularity  of  the  potash  is  shown  along  the  line  where  the 
sanidine  disappears  and  some  one  or  more  of  the  lime-soda  feldspars  become 
the  predominant  species,  whereas  with  the  soda  no  such  break  is  noticeable. 
In  the  liquid  mass,  under  influences  very  little  understood,  the  material 
forming  ferro-magnesian  minerals  draws  apart  from  the  alkalies  and  excess 
of  soda,  the  result  of  which  is  to  produce  separate  magmas  differing  widely 
in  chemical  composition. 

Common  Source  of  Lavas.— In  the  preceding  pages  all  the  extravasated  lavas 
have  been  considered  as  belonging  to  one  or  the  other  of  two  distinct 
magmas,  yet  it  is  impossible,  notwithstanding  they  are  so  sharply  con- 
trasted in  certain  fundamental  structural  characters,  not  to  recognize  the 
fact  that  both  magmas  stand  in  the  closest  relationship  to  each  other.  The 
similarity  in  mineral  development  as  they  approach  each  other  in  chemical 
constitution,  the  gradual  changes  in  the  relative  proportions  of  the  oxides  of 
the  different  elements  throughout  the  entire  range  of  lavas,  show  how  close 
a  connection  exists  between  them.  An  equally  strong  argument  is  found 
in  their  geological  distribution,  where  the  rhyolite  occurs  closing  up  the 
vents  occupied  by  the  feldspathic  magma  and  at  the  same  time  breaking 
out  as  the  earliest  eruptions  along  fissures  which  later  served  as  channels 
for  the  pyroxenic  magma.  The  loci  of  eruption  of  both  magmas  have  been 
shown  to  be  in  close  proximity  to  each  other,  and  some  of  the  most  acid 
and  most  basic  lavas,  so  far  as  external  evidence  can  determine,  not  only 
reached  the  surface  along  the  same  great  fractures,  but  actually  used  the 
same  conduits  at  a  number  of  localities. 

To  the  writer,  after  studying  all  the  facts,  it  seems  impossible  to  regard 
these  differentiated  volcanic  products  otherwise  than  as  belonging  orig- 
inally to  one  and  the  same  body  of  molten  material;  in  other  words,  they 
were  derived  from  a  common  reservoir.  To  conceive  of  such  a  separation 


268  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

from  an  earlier  primordial  molten  mass  is  no  more  difficult  than  to  conceive 
of  the  breaking  up  of  the  feldspathic  magma  into  a  homblende-mica- 
andesite  and  a  rhyolite  group,  and  the  latter  has  been  shown  to  take 
place,  so  far  as  it  is  possible  to  demonstrate  it  from  surface  evidences, 
along  fissure  planes  through  which  the  lavas  issued.  The  original  magma 
separated  into  a  heavier  and  a  lighter  portion,  the  groundmass  structure 
of  the  two  being  fundamentally  different.  It  will  be  borne  in  mind 
that  the  earlier  magma  consisted  of  a  groundmass  made  up  of  an  aggre- 
gation of  feldspar  and  quartz  grains,  through  which  were  disseminated 
porphyritic  secretions  of  hornblende  and  mica,  but  no  pyroxene,  except 
in  a  few  instances  of  pyroxene  microlites  in  the  groundmass  of  some 
varieties  of  audesite.  The  later  magma  consisted  of  a  groundmass  com- 
posed of  lath-shaped  lime-soda  feldspars  and  pyroxene  microlites,  so  intri- 
cately interwoven  as  to  form  the  so-called  felt-like  structure  characteristic 
of  pyroxene-andesite,  through  which  were  scattered  the  heavier  ferro- 
magnesian  minerals  already  described. 

History  of  Volcanic  Action.— The  geological  history  of  volcanic  action  at 
Eureka  during  Tertiary  time  is  in  many  respects  simple  and,  after  a  careful 
study  of  its  details,  easily  deciphered.  There  are  among  the  lavas  no  masses 
of  coarsely  crystalline  rocks  slowly  cooled  beneath  the  surface  under 
physical  conditions  different  from  those  usually  found  accompanying  extru- 
sive flows.  No  powerful  displacements  have  brought  into  juxtaposition 
igneous  rocks  of  different  ages,  crystalline  structure  and  mineral  composi- 
tion, and  although  faulting  attending  extravasation  doubtless  did  occur  it 
was  not  of  a  kind  to  obscure  geological  structure.  Again,  the  sequence  of 
events  was  not  complicated  or  broken  by  long  intervals  of  activity  and 
rest  through  successive  geological  epochs  during  which  an  older  and  a 
younger  series  of  eruptions  took  place;  but  on  the  contrary  the  lavas  were 
apparently  poured  out  under  very  similar  physical  conditions  from  the 
beginning  to  the  end  of  volcanic  action.  In  coming  to  the  surface  these 
lavas  were  not  forced  upward  as  one  continuous  eruption  or  rapid  series  of 
eruptions,  but  were  the  result  of  a  succession  of  overflows  accumulating 
slowly,  although  at  times  spasmodically,  along  lines  of  volcanic  activity 
coincident  with  lines  of  orographic  displacement.  The  material  thus  poured 


HISTORY  OF  VOLCANIC  ACTION.  269 

out  gradually  underwent  changes  in  mineral  composition  offering  a  great 
variety  of  volcanic  products  of  which  the  relative  age  and  order  of  succes- 
sion of  typical  lava  flows  have  been  clearly  established.  It  has  also  been 
demonstrated  that  throughout  this  entire  series  of  lavas  the  range  in  silica 
amounts  to  about  25  per  cent,  a  range  which  is  quite  as  wide  as  is  ordinarily 
found  in  most  centers  of  eruption,  even  where  the  volume  of  lavas  thrown 
out  has  been  vastly  greater  and  the  duration  of  volcanic  energy  far  longer. 
The  succession  of  events  throughout  the  volcanic  period  presents  a  con- 
tinuous chapter  of  geological  history  complete  in  itself  with  the  rise,  cul- 
mination and  dying  out  of  eruptive  energy.  So  far  as  ultimate  chemical 
composition  of  both  acid  and  basic  rocks  is  concerned  it  furnishes  a  com- 
plete cycle  of  volcanic  products. 

Probably  the  feldspathic  and  pyroxenic  lavas  do  not  approach  each 
other  in  their  tenure  of  silica  within  2-25  per  cent,  at  least  no  body  of  rock 
or  lava  stream  is  known  which  indicates  a  closer  coming  together  of  the 
two  magmas.  In  chemical  composition  and  mineral  development  the  earli- 
est eruptions  of  both  magmas  resemble  each  other  closest,  but  from  this 
common  ground  they  differentiate  steadily  until  the  feldspathic  lavas  reach 
the  extreme  acidic  and  the  pyroxenic  the  extreme  basic  end  of  their  respec- 
ive  series.  The  former  and  earlier  magma  exhibits  in  the  overflows  a  con- 
stantly increasing  acidity  through  a  range  of  11  per  cent  of  silica,  and  the 
latter  an  increasing  basicity  with  a  falling  away  in  silica  of  13  per  cent,  the 
point  of  separation  of  the  two  magmas  being  nearly  midway  between  the 
extremes  in  composition. 

Exceptional  lavas  in  other  localities  may  carry  somewhat  more  silica 
than  those  thrown  out  at  Eureka,  but  it  is  doubtful  if  flows  of  any  consid- 
erable size  exceed  those  of  Rescue  Canyon  in  acidity  by  more  than  2  per 
cent  unless  accompanied  by  secondary  alterations  or  infiltration  products. 
Obsidians  are  reported  as  carrying  78  per  cent  of  silica,  but  for  the  most 
part  these  highly  acidic  glasses  fall  within  the  limits  assigned  to  normal 
rhyolites.  Basalts  somewhat  richer  in  oliviue  and  magnetic  iron  are  by 
no  means  uncommon  elsewhere,  but  these  extreme  basic  varieties  have  not 
as  yet  been  recognized  within  the  Great  Basin.  Not  only  as  regards  the 
range  in  silica,  but  for  all  other  essential  elements  entering  into  the  original 


270  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

composition  of  magmas,  this  series  of  lavas  may  be  taken  as  representative 
of  many  others  in  widely  separated  regions  throughout  the  world.  To  the 
lavas  of  Hungary  they  show  very  close  resemblance. 

Beginning  with  the  hornblende-andesite  the  feldspathic  magma  became 
gradually  more  siliceous  until  the  close  of  the  rhyolitic  eruptions  without 
any  abrupt  break  in  the  outpourings  or  the  intervention  of  any  percepti- 
ble change  in  geological  conditions.  It  seems  impossible,  therefore,  to 
consider  these  lavas  in  any  other  light  than  as  a  continuous  succession  of 
flows,  interrupted  only  by  time  intervals  of  longer  or  shorter  duration. 
Notwithstanding  these  gradual  transitions,  certain  type  rocks  prevail  to  a 
far  greater  degree  than  others,  both  as  regards  bulk  and  distribution,  nota- 
bly the  hornblende-mica-andesite  and  the  Pinto  Peak  variety  of  the  rhyo- 
lite,  the  two  standing  out  prominently  as  the  principal  eruptions  of  the 
feldspathic  series.  The  dacites  are  greatly  limited  in  their  bulk,  and  the 
same  is  true  of  all  rocks  of  intermediate  composition,  the  greater  part  of 
them  being  easily  classed  under  one  or  the  other  of  the  natural  groups. 

The  earliest  outbursts  along  different  profound  fissure  planes  have 
not  necessarily  been  identical  in  composition  or  synchronous  in  time.  Along 
some  of  these  the  first  overflows  observed  are  hornblende-mica-andesite, 
in  others  highly  siliceous  andesitic  pearlites,  in  still  others  dacites,  and  in 
several  of  them  rhyolites,  but  in  no  single  instance,  whatever  may  have 
been  the  nature  of  the  earliest  lava  poured  out,  has  a  more  basic  member 
of  the  feldspathic  series  been  recognized  as  breaking  out  along  the  same 
fissure.  It  is  as  if  certain  of  these  fissures  were  opened  by  the  forcing 
upward  of  the  lavas  at  different  periods  of  eruptive  energy  and  the  vents 
filled  by  a  magma  of  definite  composition  at  that  time  coming  to  the  surface 
simultaneously  through  all  the  fissures.  It  is  also  worthy  of  note  that 
along  the  meridional  faults  the  andesitic  material  for  the  most  part  broke 
out  at  the  northern  ends,  the  lavas  in  general  growing  more  acidic  toward 
the  south.  Furthermore,  certain  fissures  becoming  filled  and  choked  by 
cooling  and  crystallization  have  prevented  the  more  acidic  lavas  from  find- 
ing an  outlet  at  the  surface  along  the  same  line  where  the  earlier  portions 
of  the  molten  mass  broke  out. 


HISTORY  OF  VOLCANIC  ACTION.  271 

When  it  comes  to  the  pyroxenic  magma  it  is  found  to  break  out  and 
follow  the  sinuous  lines  of  fracture  previously  followed  by  rhyolitic  lavas. 
In  some  instances  they  present  the  appearance  of  actually  employing  the 
identical  conduits  used  by  the  feldspathic  magma.  In  this  way  the  rhy- 
olite  plays  a  most  important  part,  not  only  as  a  connecting  link  between 
the  feldspathic  and  pyroxenic  magmas  in  respect  to  sequence  of  flow,  but 
still  more  as  regards  geological  distribution  and  mode  of  occurrence.  Too 
much  stress  can  not  be  laid  upon  the  fact  already  mentioned,  that  the  rhy- 
olites  were  the  last  to  break  out  along  the  vents  occupied  by  the  hornblende- 
andesite  and  the  first  to  reach  the  surface  along  the  same  lines  of  fracture 
which  were  afterward  used  by  the  basalts  of  the  pyroxenic  magma.  That 
these  basic  lavas  may  have  occasionally  forced  open  new  vents  for  them- 
selves is  quite  possible,  but  the  greater  number  of  outbursts  followed  the 
same  grand  fractures  as  the  earlier  highly  acidic  magmas  which  border  the 
elevated  orographic  block  of  Silverado  and  County  Peak.  Richmond 
Mountain,  as  already  pointed  out,  may  have  reached  the  surface  through  a 
separate  and  wholly  independent  vent,  but  it  is  so  vast  and  its  overflows 
cover  so  large  an  area  that  it  is  impossible  to  determine  the  position  of  its 
vent  or  vents  and  their  precise  relation  to  the  earlier  rhyolite.  It  must  be 
borne  in  mind,  however,  that  it  breaks  out  at  the  junction  of  two  grand 
lines  of  faulting,  coming  up  from  the  south  on  opposite  sides  of  a  great 
uplifted  mountain  mass.  The  earliest  flows  of  the  pyroxenic  magma 
resembled  those  of  the  feldspathic  magma,  in  so  far  as  they  carry  the  same 
ferro-magnesian  silicates  as  porphyritic  secretions.  On  the  other  hand,  they 
are  sharply  contrasted  by  an  andesitic  habitus  of  the  groundmass,  which, 
however,  had  been  slightly  foreshadowed  by  a  groundmass  carrying  pyrox- 
ene microlites,  shown  in  the  basic  pearlite  from  the  south  end  of  Carbon 
Ridge,  where  the  rock  occurs  as  the  earliest  eruption  at  that  locality,  fol- 
lowed by  a  series  of  feldspathic  lavas,  closing  with  rhyolite. 

Following  the  great  body  of  pyroxene  andesite  came  lavas  intermedi- 
ate in  composition  between  them  and  basalt,  breaking  through  and  over- 
lying the  less  basic  varieties.  Some  of  these  are  allied  to  the  earlier  flows, 
while  others  show  a  decided  tendency  to  transition  into  basalt.  Most  of 
them  are  related  geologically  either  with  the  later  basaltic  eruptions  or 


272  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

stand  alone,  having  broken  through  rhyolite.  A  large  portion  of  the  rock 
masses  designated  as  pyroxene-andesite  would  hardly  be  classed  as  typical 
rock  of  that  natiiral  group,  and  the  same  may  be  said  of  many  of  the 
basaltic  flows  which  are  far  too  rich  in  silica  and  wanting  in  olivine  to  be 
regarded  as  normal  basalt.  It  is  probable  that  many  modern  volcanoes 
would  show  the  same  wide  range  iu  basic  lavas  as  is  developed  in  the  region 
of  Richmond  Mountain. 

Throughout  a  wide  range  in  composition  and  over  an  extended  geo- 
graphical area  the  pyroxenic  magma  fails  to  show  the  tendency,  so  strongly 
marked  in  the  feldspathic  magma,  to  separate  into  well  defined  natural 
groups,  nor  is  the  evidence  by  any  means  clear  that  during  the  period  of 
extravasation  a  steady  increase  in  the  basicity  of  the  lava  took  place  with- 
out occasional  oscillations  in  composition.  Nevertheless,  it  is  evident  that 
whatever  oscillations  there  were  must  have  been  confined  within  very  narrow 
limits  -and  restricted  to  lavas  of  intermediate  composition  between  pyroxene- 
audesite  and  basalt.  No  pyroxene -andesite  dikes  have  been  observed  pen- 
etrating either  the  basalts  or  the  intermediate  lavas. 

It  seems  evident  from  field  observations  that  there  were  no  abrupt 
alterations  of  feldspathic  and  pyroxenic  lavas  after  the  appearance  of  the 
earliest  pyroxene-andesite. 

Speculative  Theories.— It  does  not  come  within  the  scope  of  this  chapter, 
which  is  mainly  devoted  to  a  presentation  of  observed  facts,  to  enter  upon  a 
full  discussion  of  the  speculative  theories  advanced  by  geologists  to  account 
for  the  condition  of  the  molten  masses  beneath  the  surface,  nor  the  physical 
causes  leading  to  their  Reparation  into  the  varied  products  found  either  as 
interbedded  sheets  and  laccolites  within  the  superficial  crust  of  the  globe, 
or  poured  out  upon  the  surface  as  extrusive  lavas.  Yet,  at  the  same  time, 
after  having  devoted  so  much  study  to  the  constitution  of  the  different 
lavas  and  their  order  of  succession,  this  chapter  would  be  incomplete  with- 
out calling  attention  to  the  importance  of  the  phenomena  presented  at 
Eureka  and  pointing  out  the  bearing  of  the  observed  facts  upon  the  prob- 
lems offered  in  volcanic  regions  elsewhere.  Without  entering  upon  a  review 
in  detail  or  a  critical  discussion  of  the  opinions  held  by  others  who  have 
considered  these  speculative  matters,  it  is  necessary  to  recall,  briefly,  the 


BTINSEN'S  VIEWS.  273 

views  expressed  in  the  more  important  contributions  to  the  literature  on  the 
subject. 

Bunsen's  views.— Bunsen,  after  a  visit  to  Iceland,  where  he  laboriously 
studied  the  volcanic  phenomena  displayed  on  a  grand  scale,  conceived 
the  idea  of  two  distinct  bodies  of  lava,  one  acid  and  the  other  basic,  the 
former  of  which  he  designated  as  the  normal  trachytic,  the  other  as  the 
normal  pyroxenic  magma.  He  was  disposed  to  regard  all  volcanic  products 
intermediate  in  composition  between  these  types  as  admixtures  in  varying 
proportions  derived  from  two  distinct  foci  of  eruption,  the  relative  propor- 
tions of  each  depending  in  great  part  upon  the  intensity  of  eruptive 
energy.  He  sought  to  apply  his  views  to  all  other  volcanic  regions,  citing 
as  an  identical  mode  of  occurrence  the  table-land  of  Armenia.1  The  grand 
division  of  volcanic  products  into  acid  and  basic  lavas  has  been  received 
by  most  vulcanologists,  but  his  theories  to  account  for  the  very  varied  con- 
stitution of  volcanic  rocks  has  not  obtained  the  same  general  acceptance. 
In  this  chapter  the  writer  adopts  the  views  of  Bunsen  as  regards  two  great 
groups  of  lavas,  but  differs  with  him  as  to  the  origin  of  the  varied  transi- 
tion products  of  eruption. 

The  writer  has  used  the  expression  feldspathic  magma  in  preference  to 
trachytic  magma,  as  the  former  is  a  mineralogical  term  contrasting  sharply 
with  the  expression  pyroxemc  magma.  This  is  rendered  all  the  more  neces- 
sary since  the  word  trachytic  now  possesses  a  different  signification  from 
what  it  did  at  the  time  when  it  was  first  employed  by  the  German  scientist. 
Typical  trachytes  are  somewhat  rare  and  confined  to  restricted  areas,  since 
many  of  the  rocks  formerly  considered  as  trachyte  have  been  found  to  be 
characterized  by  plagioclastic  feldspars,  and  hence  more  properly  come  under 
the  head  of  andesite.  This  is  the  case  with  the  feldspathic  rocks  of  Ice- 
land, which  Bunsen  investigated  and  upon  which  he  bases  his  conclusions. 

Durocher's  Theories.— Durocher,3  after  studying  the  composition  and  petro- 
graphical  characters  of  a  large  number  of  crystalline  rocks,  endeavored  by 
ingenious  and  somewhat  complex  theories  to  establish  universal  laws  to 
account  for  the  variations  observed  in  crystalline  rocks  of  all  ages  and 

1  Ueber  die  Proeesse  AVI  vulkanischen  Gesteinsbildung  Islands.  Poggendorf  s  Anualen,  1851, 
Band  83,  pp.  197-272. 

3 Essai  do  petrologie  compare.     Ann.  d.  mines,  Paris,  5th  ser.,  1857,  Tome  xi,  pp.  217-259. 
MON  XX 18 


274  GEOLOGY  OF  THE  EUKEKA  DISTRICT. 

of  every  possible  mode  of  occurrence.  He  followed  Bunseii  in  accepting 
the  theory  of  both  an  acid  and  basic  magma,  but  regarding  them  as 
parts  of  the  same  body  of  lava.  In  an  appendix  to  his  paper1  he 
admits  the  possibility  in  certain  cases  of  a  mingling  of  both  types,  but 
objects  to  the  hypothesis  of  Bunsen  as  altogether  too  broad  a  general- 
ization. That  part  of  Durocher's  hypothesis  which  possesses  the  most 
originality  and  upon  which  he  places  the  most  stress  to  account  for  the 
differences  in  the  mineralogical  character  of  lavas  has  been  designated  the 
liquation  process  applied  to  igneous  rocks.  His  conclusions,  based  largely 
upon  chemical  analyses,  were  not  substantiated  by  any  array  of  facts  or 
observations  from  any  one  center  of  volcanic  energy.  Durocher  was  dis- 
posed to  regard  certain  lavas  as  differentiated  products  obtained  by  the 
breaking  up  of  a  magma  by  processes  comparable  to  the  separation  and 
segregation  of  metals  in  a  bath  containing  several  metallic  substances  in  a 
state  of  fusion,  the  theory  being  based  upon  well  recognized  processes  em- 
ployed in  metallurgical  establishments  for  the  concentration  of  gold  and 
silver  in  molten  lead.  The  views  enunciated  by  Durocher  have  met  with 
slight  recognition,  but,  although  containing  much  that  with  the  advance- 
ment of  knowledge  has  been  shown  to  be  based  upon  error,  they  are,  in 
the  opinion  of  the  writer,  full  of  the  most  valuable  suggestions  bearing  on 
the  origin  of  lavas,  and  entitled  to  far  more  consideration  than  has  generally 
been  accorded  them. 

Roth's  views.— In  1861  Justus  Roth2  published  his  hypothesis  of  "Spal- 
tung  und  Differeiizirung,"  in  which  he  elaborated  similar  views,  although  by 
no  means  identical  with  those  held  by  Durocher.  For  the  purposes  of  this 
chapter  it  is  sufficient  to  say  that  the  two  authors  are  in  accord  so  far  as 
believing  in  the  power  of  a  magma  to  split  up  during  crystallization  into 
secondary  magmas  of  different  mineralogical  composition.  Roth  regarded 
large  bodies  of  crystalline  rocks  as  "Spaltungsproducte,"  the  result  of  the 
separating  out  of  certain  groups  or  association  of  minerals  from  and  de- 
pendent upon  the  composition  of  a  primary  liquid  lava,  but  governed  by 
varying  conditions  of  pressure  and  temperature.  His  views  are  derived 

1  Op.  cit.,  p.  677. 

2  Tabellarische  Uebersicht  <ler  Gesteins  Analyseu  und  mit  kritischen  Erliiuterungen.    Berlin,  1861. 


RICHTHOFEISPS   VIEWS.  275 

from  a  careful  study  and  comparison  of  a  large  number  of  chemical  analyses 
of  crystalline  rocks  gathered  from  all  parts  of  the  world,  differing  widely  in 
their  mineralogical  development  and  structural  habit. 

Besides  these  references  to  original  contributions  the  reader  who  de- 
sires to  pursue  the  subject  still  further  will  find  an  excellent  summary  of 
the  views  of  Bunsen,  Durocher,  Roth,  and  others,  published  by  Ferdinand 
Zirkel  in  his  text-book  of  petrography.! 

Waitershausen's  Conclusion.— Sartorius  von  Waltershauseii,  after  a  careful 
investigation  of  the  lavas  of  Sicily  and  Iceland,  published  the  results  of  his 
researches  in  an  elaborate  memoir  in  which  he  presented  his  conceptions  of 
the  physical  condition  of  the  interior  of  the  earth.  His  conclusions,  so  far 
as  they  relate  directly  to  subjects  considered  here,  stated  briefly,  were  that 
between  a  superficial  cool  crust  and  a  solid  interior  there  existed  a  broad 
belt  of  fused  material  of  undetermined  thickness  which  furnished  the  source 
of  supply  for  the  lavas  poured  out  upon  the  surface.  This  material 
arranged  itself  approximately  according  to  its  density.  The  most  acid  lava, 
being  the  lightest,  was  situated  nearest  the  surface,  followed  by  that  of 
intermediate  composition  characterized  by  minerals  of  somewhat  higher 
specific  gravity,  and  terminating  finally  with  the  heaviest,  and  consequently 
most  basic,  lavas — basalts — carrying  large  amounts  of  magnetite  and  other 
iron  minerals.  He  concludes  that  in  most  instances  the  lavas  were  ejected 
in  the  order  of  their  position,  the  lightest  being  first  thrown  out,  imperfect 
separation  by  specific  gravity  being  sufficient  to  account  for  all  exceptional 
occurrences.  This  simple  and  regular  order  of  succession  met  nearly  all 
the  requirements  of  Waitershausen's  personal  observations  and  were  in 
accord  with  his  theories.2 

Richthofen's  views.— Baron  von  Richtliofeii  accepted  the  main  conclusions 
of  Waltershausen  regarding  the  physical  conditions  of  the  globe,  agreeing 
with  him  as  to  the  evidences  of  a  liquid  mass  lying  between  a  solid 
interior  and  a  superficial  outer  crust.  This  liquid  mass  was  acid 
near  the  surface,  basic  beneath,  with  the  intermediate  transition  lavas 
between  them.  He  traveled  extensively  in  the  volcanic  regions  of  Europe 

1  Lehrlmch  der  Petrographie.     Bonn,  erster  Band,  1866,  pp.  453-473. 

'-  Uebev  die  vulkanischen  Gesteine  in  Sicilieu  und  Island  und  ihre  submarine  Umbildung,  Qot- 
tingt-n,  185i<. 


276  GEOLOGY  OF  THE  EUEEKA  DISTRICT. 

and  western  America,  studying  the  development  of  volcanic  rocks.  He 
devoted  special  attention  to  the  laws  governing  the  mode  of  occurrence  of 
the  different  natural  groups  into  which  he  divided  all  igneous  rocks,  and 
the  relations  of  these  groups  to  each  other,  being  more  interested  in  the 
geological  problems  than  in  the  precise  chemical  composition  of  the 
extruded  products.  As  a  result  of  his  observations  in  the  field,  he  was 
impressed  by  the  great  similarity  in  the  nature  of  lavas  in  widely  sepa- 
rated regions  and  the  uniformity  in  the  order  of  their  succession.  He  found, 
however,  that  this  succession  was  by  no  means  as  simple  as  suggested  by 
Waltershausen,  nearly  every  volcanic  region  which  he  visited  presenting 
abrupt,  but  similar,  alternations  from  acid  to  basic  rocks,  at  first  sight  not 
readily  explained.  Richthofen's  final  conclusions  were  published  in  an 
admirable  and  remarkably  suggestive  memoir  presented  to  the  California 
Academy  of  Sciences,1  in  which  he  gives  what  he  considers  to  be  the 
natural  law  of  the  sequence  of  massive  eruptions  applicable  to  all  centers 
of  volcanic  energy.  As  his  conclusions  were  based  largely  on  observations 
made  in  California  and  the  western  edge  of  the  Great  Basin,  they  are  of 
more  than  ordinary  interest  for  purposes  of  comparison  with  results  since 
obtained  by  the  investigations  at  Eureka. 

The  natural  order  of  succession  of  massive  eruptions  as  laid  down 
by  Richthofen  is  as  follows:2 

1.  Propylite. 

2.  Andesite. 

3.  Trachyte. 

4.  Ehyolite. 

5.  Basalt. 

This  law  of  succession  as  enunciated  by  Richthofen  is  far  more  com- 
plex than  the  simple  regular  order  suggested  by  Waltershausen,  as  it 
supposes  the  breaking  out,  first  of  all,  of  intermediate  lavas  represented  by 
propylites  and  andesites,  followed  by  others  of  varying  composition, 
but  more  acidic  than  the  latter  and  belonging  to  the  order  trachytes.  The 
trachytes  were  succeeded  by  a  still  more  acid  series  of  lavas,  and  then  the 
closing  of  eruptive  energy  by  an  abrupt  change  from  the  most  acidic  of  all 

1  Natural  system  of  volcanic  rocks.     Memoirs  of  the  California  Academy  of  Sciences,  1867,  vol.  I,  p.  36. 
a  Op.  cit.,  p.  29. 


KING'S  VIEWS.  277 

lavas,  the  rhyolites,  to  the  most  basic  of  all,  the  basalts.  From  this  order 
he  nowhere  recognized  any  deviation.  Accepting  the  hypothesis  of  Walter- 
shausen  as  regards  a  liquid  interior  and  the  nature  of  the  molten  mass,  he 
seeks  to  account  for  the  remarkable  alternations  observed  in  lavas  upon  the 
surface  of  the  globe  by  supposing  changes  to  take  place  in  the  physical 
conditions  governing  the  emission  of  lavas  which  would  from  time  to  time 
elevate  or  depress  the  loci  of  eruption.  These  changing  conditions  were 
universal,  producing  similar  results  in  volcanic  centers  all  over  the  world, 
but  not  necessarily  contemporaneous  in  time.  He  says: 

It  appears  that  after  the  ejection  of  the  chief  bulk  of  andesite,  when  other  pro- 
cesses ending  in  the  opening  of  fractures  into  the  basaltic  region  were  being  slowly 
prepared  in  depth,  the  seat  of  eruptive  activity  ascended  gradually  to  regions  at  less 
distance  from  the  surface.1 

clarence  King's  views.— As  a  part  of  the  report  upon  the  Geological 
Exploration  of  the  Fortieth  Parallel,  Mr.  Clarence  King  published  in  1878 
the  results  of  his  researches  upon  the  genesis  of  lavas  as  shown  by  their 
occurrences  in  the  field  of  his  observations  in  the  Great  Basin.  As  regards 
the  law  of  succession,  his  views  are  for  the  most  part  in  accord  with  those 
of  Richthofen,  he  going,  however,  still  further  and  finding  a  much  more 
intricate  system  in  the  alternations  from  acid  to  basic  rocks.  He  finds  an 
acid,  a  neutral,  and  a  basic  member  in  each  natural  group  or  order  which 
he  designates  by  specific  names,  each  member  having  a  definite  mineral 
composition  and  a  fixed  place  in  the  order  of  succession.  To  these  modifi- 
cations proposed  to  Richthofen's  order  he  adds  another  still  more  radical, 
in  respect  to  classification,  uniting  rhyolite  and  basalt  under  one  head,  to 
which  he  applies  a  new  designation,  "Neolite,"  these  two  types  of  lava 
constituting  the  acid  and  basic  subdivisions  of  this  natural  group,  having 
the  same  relative  value  as  andesite  and  trachyte.  The  sequence  of  lavas  as 
recognized  by  Mr.  King  is  as  follows  :2 

1  Op   cit.,  p.  58. 

*U.  S.  Geol.  Explor.  of  the  Fortieth  Parallel,  vol.  i,  Systematic  Geology,  p.  690. 


278  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

Natural  succession  of  volcanic  rocks. 

Order.  Subdivision. 

1.  Propylite «.  Hornblende-propylite. 

b.  Quartz-propylite. 

c.  Augite-propylite. 

2.  Andesite a.  Hornblende-andesite. 

b.  Quartz-andesite  (Dacite). 

c.  Augite-andesite. 

3.  Trachyte a.  Hornblende-plagioclase-trachyte. 

b.  Sanidine-trachyte  (quartziferous). 

c.  Augite-trachyte. 

4.  Neolite a.  Rhyolite. 

b.  Basalt. 

This  presents  a  much  more  complex  system  and  could  hardly  be 
accepted  upon  the  simple  conditions  of  a  uniform  and  widespread  liquid 
mass,  as  held  by  Waltershausen  and  modified  by  Richthofen,  in  requiring 
frequent  elevation  and  depression  of  the  loci  of  eruption  hi  accord  with  the 
changes  in  the  composition  of  the  lava  thrown  out  at  the  surface.  Mr. 
King  is  fully  aware  of  the  many  physical  obstacles  encountered,  and 
explains  the  many  oscillations  and  abrupt  alternations  in  the  volcanic 
products  which  his  system  calls  for  by  a  carefully  considered  hypothesis  of 
his  own,  quite  at  variance  with  the  views  advanced  by  his  predecessors. 
In  place  of  a  broad  belt  or  magma  of  liquid  lava  encircling  the  earth 
beneath  the  sedimentary  crust,  he  holds  to  the  opinion  of  local  reservoirs  of 
molten  matter  within  the  superficial  crust,  each  of  his  orders  being  the 
product  derived  from  one  of  these  reservoirs,  or,  as  he  calls  them, 
"  extremely  localized  and  only  temporarily  existing  pools  of  fusion." 
He  says: 

Under  my  hypothesis,  by  which  fusion  is  the  temporary  result  of  erosion,  each 
one  of  Richthofen's  orders,  with  its  acidic  and  pyroxenic  members,  would  be  considered 
as  the  product,  of  a  single  ephemeral  lake.  A  period  of  erosion  under  this  conception 
would  result  in  the  formation  of  a  lake.  The  cessation  of  erosion,  either  from  climatic 
causes  or  from  the  degradation  of  centers  of  erosion,  would  place  a  limit  to  the 
expansion  in  depth  of  fusion;  in  other  words,  would  define  the  time  limits  and  the 
vertical  expansion  of  the  lake.1 

'Op.  cit.,  p.  716. 


ABSENCE  OF  PROPYLITE.  279 

» 

His  views,  which  can  not  well  be  abridged  here,  will  be  found  admira- 
bly stated  in  his  chapter  devoted  to  a  discussion  of  the  genesis  of  volcanic 
species,  in  which  he  treats  of  geological  causes  leading  to  the  formation 
of  local  lakes  of  lava. 

Later  observations.— Since  the  publication  of  King's  memoir  the  study  of 
volcanic  rocks  has  progressed  with  rapid  strides,  and  nowhere  have  they 
been  investigated  with  more  untiring  energy  than  in  the  Cordillera  of 
North  America.  Notwithstanding  our  knowledge  of  the  rocks  of  the 
Washoe  District  and  the  Coinstock  Lode,  derived  from  the  works  of  Rich- 
thofen  and  King,  later  study  of  them,  aided  by  methods  of  microscopical 
research,  has  developed  fresh  points  of  interest  bearing  upon  their  order  of 
succession  and  mutual  relations.  After  a  thorough  examination  of  the 
propylites,  Mr.  George  F.  Becker1  has  shown  that  they  can  not  be  separated 
from  the  andesites  as  an  independent  rock  species  based  upon  any  mineral- 
ogical  distinctions,  since  the  peculiar  habitus  of  the  propylite  is  due  to 
chemical  change  and  decomposition  of  the  constituent  minerals.  Moreover, 
the  propylites  and  andesites  are  found  to  pass  into  each  other  by  gradual 
transitions. 

Hague  and  Iddings,2  in  the  course  of  their  examination  of  the  Washoe 
rocks,  confirmed  the  results  of  Mr.  Becker  so  far  as  the  identity  of  the 
propylite  and  andesite  is  concerned,  and  also  failed  to  see  any  geological 
evidences  of  a  preandesitic  eruption. 

Similar  views  as  regards  the  independence  of  propylite  are  now  main- 
tained by  nearly  all  petrographers  who  have  given  much  thought  to  the 
subject  or  who  attempt  to  classify  volcanic  rocks  upon  either  a  structural  or 
mineralogical  basis.3 

1  Geology  of  the  Comstock  Lode  and  the  Washoe  District,  Washington,  1882. 

*On  the  development  of  crystallization  in  the  igneous  rocks  of  Washoe,  Nevada;  with  notes  on 
geology  of  the  district.  Bull.  U.  S.  Geol.  Survey,  No.  17,  Washington,  1885. 

3  Since  this  chapter  was  written  Prof.  J.  W.  Judd  has  published  an  admirable  paper  on  "  The 
Propylites  of  the  Western  Isles  of  Scotland,  and  their  Relation  to  the  Andesites  and  Diorites  of  the 
District."  He  revives  the  use  of  the  term  propylite,  but  in  the  strict  sense  suggested  by  Rosenbusch, 
regarding  it  simply  as  a  "pathological  variety"  of  andesite.  His  detailed  descriptions  identify  the 
Scottish  rocks  with  similar  rocks  found  iu  Hungary.  From  his  description  it  would  be  difficult  to 
distinguish  thorn  in  any  particular  from  the  altered  andesites  of  the  Washoe  District  in  the  Virginia 
Range.  They  even  show  the  development  of  metallic  sulphides.  Quart.  Journ.  Geol.  Soc.,  vol.  XLVI, 
pp.  341-382.  London,  1890. 


280  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

The  same  writers  have  demonstrated  the  nonexistence  of  trachyte  as 
one  of  the  natural  divisions  of  volcanic  lavas  in  the  Great  Basin,  the 
occurrence  of  orthoclase  rocks  free  from  quartz  secretions  being  almost 
unknown  in  that  region.  These  recent  advances  in  our  knowledge  of  vol- 
canic rocks  tends  to  simplify  the  law  of  sequence  so  far  as  their  occurrence 
in  the  Great  Basin  is  concerned,  since  two  of  the  groups,  the  propylite  and 
the  andesite,  as  laid  down  by  Richthofen,  have  been  merged  into  one, 
and  the  trachytes  either  relegated  to  some  variety  of  andesitic  lavas  or 
placed  among  quartz-bearing  rocks,  either  dacite  or  rhyolite.  The  impor- 
tance of  these  observations  lies  in  the  fact  that  there  is  no  interpolation  of  a 
strongly  alkaline  magma  in  the  series  of  lavas,  and  that  andesitic  lavas  pass 
over  directly  into  rhyolite.  Not  only  in  the  Great  Basin  but  in  many  other 
regions  as  well,  rhyolite  is  far  more  closely  related  to  andesites  derived 
from  a  feldspathic  magma  than  to  trachytes. 

Having  thus  briefly  reviewed  the  literature  bearing  upon  the  genesis 
of  lavas  and  their  order  of  succession,  it  becomes  a  matter  of  much  interest 
to  see  how  far  the  facts  observed  in  a  carefully  studied  and  surveyed  region 
like  Eureka  are  in  accord  with  the  Adews  expressed  by  the  eminent  writers 
quoted,  since  it  is  only  by  the  accumulation  of  vast  amount  of  evidence 
from  many  widely  separated  fields  that  we  can  hope  to  attain  anything  like 
definite  laws  governing  the  mutual  relations  of  igneous  rocks. 

In  only  one  other  i-egion  of  the  Great  Basin  have  volcanic  phenomena 
been  investigated  in  a  manner  at  all  comparable  with  Eureka  and  that  one 
the  much  discussed  area  of  the  Washoe  District.  At  Washoe  the  conditions 
are  in  some  respects  very  different,  volcanic  activity  having  extended 
through  a  longer  period  of  time.  The  coarse  crystalline  rocks  which  form 
the  long  slopes  of  Mount  Davidson  do  not  make  their  appearance  at  Eureka, 
and  for  that  matter  are  wanting  over  the  greater  part  of  the  Nevada  plateau. 
They  belong  to  an  earlier  period,  forming  a  distinct  chapter  in  the  Tertiary 
history  of  volcanic  action. 

The  earliest  eruptions  at  Eureka  may  be  correlated  with  the  horn- 
blende-mica-andesite  of  Washoe  (trachytes  of  Richthofen  and  King  and 
later  hornblende-andesites  of  Becker)  in  mineral  composition  and  structural 
features.  They  may  be  regarded  from  the  point  of  view  of  this  chapter  as 


EUEEKA  AND  WASHOE.  281 

synchronous  in  age,  since  the  succession  of  all  subsequent  lava-flows  for  the 
feldspathic  rocks  in  both  localities  may  be  said  to  be  the  same — hornblende- 
mica-andesite,  dacite,  rhyolite.  Analyses  show  the  homblende-mica-ande- 
site  rocks  of  Eureka  to  carry  slightly  more  silica  than  the  corresponding 
rocks  at  Washoe,  the  most  acid  members  of  this  group  from  the  latter  locality, 
coming  just  within  the  range  of  the  basic  members  of  the  series  at  Eureka. 

When  it  comes  to  the  pyroxenic  rocks  following  the  rhyolite  the 
sequence  of  events  does  not  appear  so  clearly  established  at  Washoe,  as 
there  no  such  grand  exposui-es  occur  as  at  Eureka.  In  the  immediate 
region  of  the  Comstock  Lode  only  a  few  isolated  patches  of  basalt  are 
exposed.  Small  outbursts  of  pyroxene-andesite,  similar  to  those  of  Rich- 
mond Mountain,  have  broken  out  only  a  short  distance  from  Mount  David- 
son, but  the  relations  between  these  two  pyroxenic  lavas  are  unknown.  A 
few  miles  northward  in  the  same  range  of  mountains  large  flows  of 
both  pyroxene-andesite  and  basalt  may  be  seen  superimposed  upon  horn- 
blende-mica-andesite  and  rhyolite.  Taken  together  the  Washoe  District 
and  the  region  of  Truckee  Canyon  present  a  sequence  of  lavas  and  a 
geological  history  of  volcanic  events  similar  to  that  found  at  Eureka. 

The  subjoined  table  presents  a  seiies  of  twelve  chemical  analyses  rep- 
resenting the  volcanic  rocks  of  Washoe  arranged  according  to  their 
basicity :' 

1  On  the  development  of  crystallization  in  the  igneous  rocks  of  Washoe,  Nevada;   with  notes  on  the 
geology  of  the  region.     Bull.  U.  S.  Geol.  Survey,  No.  17,  p.  33. 


282 


GEOLOGY  OF  THE  EUEEKA  DISTRICT, 


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SEQUENCE  OF  LAVAS.  283 

Analyses  Nos.  n  to  vn,  inclusive,  represent  Tertiary  rocks  older  than 
any  found  at  Eureka,  but  from  Nos.  vinto  xn,  inclusive,  together  with  Xo. 
i,  they  correspond  fairly  well  to  similar  lavas  at  the  latter  locality.  In  this 
table,  however,  pyroxene-andesites  similar  to  those  of  Richmond  Mountain 
and  of  the  same  geological  position,  associated  with  basalts  and  later  than 
the  rhy elites,  were  not  shown,  for  the  reason  already  stated:  that  they  lie 
beyond  the  limits  of  the  mining  districts. 

Nowhere  else  between  the  Wasatch  and  Sierra  have  the  lavas  been  so 
carefully  mapped,  and  only  in  a  few  places  do  they  appear  so  varied  and 
complete.  In  many  centers  of  eruption,  even  where  the  amount  of  lava 
poured  out  is  large,  certain  types  of  rock  are  wanting,  and  in  others  their 
relative  position  can  not  well  be  determined  owing  to  frequent  breaks  in  the 
continuity  of  exposures. 

The  history  of  volcanic  action  may  be  fragmeutal  and  only  partially 
recorded  in  any  one  locality,  but  throughout  the  Great  Basin,  where  the 
physical  and  geological  conditions  were  much  the  same  during  the  volcanic 
period,  it  is  probable  that  the  sequence  of  lava  will  be  found  to  be  in  accord 
in  many  places  with  the  observed  facts  at  Eureka.  As  a  center  of  eiopptive 
energy  in  Tertiary  time  the  Great  Basin  stands  out  as  a  geological  unit. 

The  earliest  lavas  erupted  at  Eureka  carry  from  65  to  67  per  cent  of 
silica  and  are  of  intermediate  composition,  in  accordance  with  the  broad 
generalization  of  Richthofen  and  the  facts  observed  by  others  else- 
where. From  this  middle  ground,  however,  the  lavas  increase  in  acidity 
until  they  attain  the  composition  of  the  extreme  acid  types.  The  latter  are 
in  turn  followed  by  lavas  that  are  also  intermediate  in  composition,  but 
which  increase  in  basicity  until  they  attain  the  extreme  basic  type  found  in 
the  later  basalt. 

Starting  from  a  magma  closely  related  in  composition,  they  differ- 
entiate in  opposite  directions  from  this  common  ground  until  they  reach  the 
extreme  type.  It  will  be  borne  in  mind  that  the  existence  of  both  an  acid 
and  a  basic  magma  at  Eureka  have  been  clearly  established,  and  to  this 
extent  conform  to  the  views  held  by  Bunsen.  Nowhere  are  the  two 
magmas  better  exhibited,  as  shown  in  their  distribution,  mode  of  occurrence, 
and  even  in  the  outlines  of  the  lava  masses,  both  types  of  rock  being  sharply 


284  GEOLOGY  OF  THE  EUKEKA  DISTEICT. 

contrasted  in  their  surface  features.  In  the  opinion  of  the  writer,  however, 
there  are  too  many  insurmountable  physical  obstacles  and  too  few  estab- 
lished facts  to  warrant  the  acceptance  of  any  theory  which  attempts  to 
account  for  the  varied  products  of  eruption  by  supposing  them  to  be  admix- 
tures from  wholly  distinct  reservoirs.  The  observed  geological  phenomena 
at  Eureka  tend  to  controvert  such  a  theory  where  the  two  magmas,  although 
in  close  proximity,  fail  to  show  any  mingling  of  products  from  separate 
reservoirs. 

Furthermore,  there  are  no  evidences  of  any  alternating  flows  of 
feldspathic  and  pyroxenic  magmas,  nor  of  oscillations  in  relative  acidity 
within  any  acid  magma,  which  would  certainly  be  the  case  had  there  been 
any  basic  material  injected  into  the  feldspathic  lava.  Within  limited  range 
any  large  outburst  of  lava  doubtless  may  display  slight  variations  in  com- 
position, but  this  also  holds  true  for  different  parts  of  the  same  flow,  and  is 
still  more  noticeable  in  pyroxenic  magmas  owing  to  the  greater  liquidity  of 
basic  lava  streams  and  the  consequent  tendency  of  the  basic  mineral  secre- 
tions to  lag  behind.  The  first  violent  explosions  after  cessations  of  activity 
might  readily  throw  out  a  lava  slightly  different  in  composition  from  the 
regular  even  flow  of  the  mass,  and  again  the  last  portions  might  vary  some- 
what in  character  from  the  great  bulk  of  molten  material. 

Evidence  is  wanting  at  Eureka  that  the  lavas  were  tin-own  out,  geolog- 
ically speaking,  from  great  distances  below  the  surface  or  from  very  vary- 
ing depths;  at  least  the  lavas  themselves  do  not  indicate  that  there  were 
any  profound  orgraphic  movements  during  the  eruptions.  Nor  is  there  any 
evidence  of  oscillation  in  depth  from  which  the  material  was  derived,  even 
if  we  accept  differences  in  specific  gravity  as  evidence  of  increase  of  dis- 
tance from  the  surface.  There  was  one,  and  only  one,  great  break  in  the 
mineralogical  character  of  the  lava.  Changes  in  specific  gravity  were 
gradual,  but  at  the  same  time  they  covered  nearly  the  entire  range  of  varia- 
tion ordinarily  found  in  volcanic  lavas.  Such  heavy  minerals  as  zircon, 
allanite,  and  garnet  occur  in  the  rocks  of  the  lowest  specific  gravity,  and  in 
the  case  of  zircons  they  are  nowhere  found  better  developed  than  in  the 
glassy  rocks  which  must  have  cooled  near  the  surface.  As  these  heavy 
infusible  minerals  were  the  first  to  crystallize  out,  they  should  have  sunk 


BASALT  AND  KHYOL1TE.  285 

to  the  bottom  if  their  position  in  the  molten  mass  was  mainly  a  question  of 
specific  gravity.  The  writer  can  not  but  regard  the  lavas  as  derived  from  a 
local  reservoir,  all  the  ejected  material  having  had  a  common  source  in 
some  primordial  magma.  The  order  of  succession  is  governed  by  far- 
reaching  physical  forces  which  may  vary  greatly  in  different  volcanic  areas, 
dependent  on  conditions  of  heat  and  pressure.  A  powerful  orographic 
movement  such  as  frequently  happens  during  a  period  of  volcanic  action 
may  be  sufficient  to  affect  the  entire  geological  conditions  in  any  eruptive 
center.  In  widely  separated  parts  of  the  world  the  extravasated  products 
are  singularly  alike,  yet  the  sequence  of  lavas  within  restricted  limits  show 
very  considerable  variation. 

Supposing  the  products  of  eruption  and  order  of  succession  to  have 
been  much  the  same  over  the  geological  province  of  the  Great  Basin,  it 
does  not  follow  that  the  same  succession  of  events  took  place  in  another 
region  where  the  geological  conditions  were  obviously  different.  Within  the 
observations  of  the  writer  instances  are  known  outside  the  Great  Basin 
where  such  an  order  of  events  not  only  did  not  take  place,  but  where  the 
mutual  relations  of  nearly  identical  lavas  exhibit  a  succession  strikingly  ar 
variance  with  the  sequence  of  flow  as  found  at  Eureka.  The  Yellowstone 
Park  may  be  cited  as  an  instance  where  the  succession  of  lavas  is  some- 
what different.  In  the  latter  locality  the  earliest  eruptions  were  of  inter- 
mediate composition,  consisting  of  hornblende-andesite  and  homblende- 
mica-andesite.  While  the  sequence  of  lavas  may  vary  owing  to  geological 
conditions,  the  laws  governing  the  differentiation  of  lava  hold  good 
everywhere. 

Basalt  and  Rhyoiite.— The  writer  accepts,  with  some  important  modifica- 
tions, the  views  of  Mr.  Clarence  King  regarding  rhyolite  and  basalt,  not 
only  as  geologically  closely  related  rocks,  but  also  as  extreme  members  of 
the  same  primordial  magma.  He  differs  from  Mr.  King  as  to  the  manner 
in  which  these  extreme  products  were  derived  from  an  earlier  molten  mass. 
It  is  nothing  against  this  view  of  their  common  origin  that  rhyolitic  out- 
bursts frequently  occur  unaccompanied  by  basalt,  or  that  basaltic  exposures 
abound  without  any  evidences  of  the  presence  of  acid  lavas.  Both  rocks 
break  out  in  the  closest  proximity  and  not  infrequently  through  the  same 


286  GEOLOGY  OF  THE  EUKEKA  DISTEICT. 

fissures,  under  precisely  similar  geological  conditions,  in  too  many  localities 
not  to  realize  their  mutual  relations.  Such  occurrences  appear  far  too 
common  the  world  over  to  permit  us  to  suppose  them  to  be  derived  from 
wholly  independent  reservoirs,  yet  everywhere  occupying  the  same  relative 
positions  with  the  basalt  superimposed  upon  the  rhyolite.  Basalt  and 
rhyolite  may  be  the  final  products  from  the  same  common  source,  but  not 
necessarily  differentiated  by  a  simple  process  of  specific  gravity  separation 
as  demanded  by  Mr.  King. 

Within  the  area  of  the  Great  Basin  there  does  not  appear  to  be  any 
rock  whose  composition  is  due  to  a  mingling  of  minerals  characteristic  of  both 
basalt  and  rhyolite.  Both  rocks,  while  they  exhibit  considerable  range  in 
chemical  composition,  always  remain  sharply  contrasted  as  regards  mineral 
constituents.  Variations  from  normal  rhyolite  carrying  orthoclase  and 
quartz  in  most  instances  show  a  transition  toward  hornblende-mica- 
andesite  through  dacite,  and  never  toward  a  pyroxenic  magma,  which 
could  hardly  be  the  case  if  the  process  was  due  wholly  to  the  dropping 
out  of  the  heavier  minerals.  Plagioclastic  feldspars  may  be  developed  in 
large  numbers,  but  they  belong  to  less  basic  species  than  those  which  char- 
acterize normal  basalt.  In  like  manner  variations  from  normal  basalt  tend 
toward  pyroxene-andesite  and  do  not  carry  orthoclase.  The  process  by 
which  the  two  magmas  are  formed  is  more  in  the  nature  of  a  differentiation 
by  molecular  change  and  changes  of  density  in  the  molten  mass  under 
varying  conditions  of  pressure  and  temperature  than  by  a  separation  of 
minerals  during  crystallization  based  upon  differences  of  specific  gravity. 
In  the  Great  Basin  and  probably  all  through  the  northern  Cordillera  con- 
ditions were  favorable  in  many  localities  for  a  complete  differentiation  of  a 
normal  magma  to  its  final  products,  rhyolite  and  basalt. 

Now,  if  we  suppose  a  magma  of  intermediate  composition,  from 
which  the  necessary  material  to  form  rhyolite  has  been  withdrawn,  the 
chemical  constitution  of  the  residue  will  depend  largely  upon  the  quantity 
of  rhyolite  produced.  If  the  quantity  of  rhyolitic  magma  thus  formed  is 
relatively  large,  the  remaining  basaltic  magma  may  be  correspondingly 
small  and  necessarily  basic  in  composition.  Again,  if  the  bulk  of  acid  or 
feldspathic  magma  which  separated  out  is  small,  there  will  remain  a  rela- 


DIFFERENTIATION  OF  LAVAS.  287 

tively  large  quantity  of  pyroxenic  magma,  but  less  basic;.  If  the  lava 
which  crystallized  out  from  this  latter  magma  upon  cooling  is  forced 
upward  to  the  surface,  it  may  consist  of  both  pyroxene-andesite  and  basalt, 
as  at  Eureka.  It  may  be  wholly  a  normal  basalt,  as  shown  in  a  number 
of  localities  in  the  Great  Basin,  or  it  may  be  largely  made  up  of  magnetite 
and  other  iron  minerals,  forming  a  basic  rock  not  yet  recognized  in  the 
Great  Basin,  but  known  elsewhere  at  several  widely  separated  places  in  the 
world.  It  is  a  matter  of  observation  in  many  localities  that  where  the  bulk 
of  rhyolite  is  excessive  the  basalt  outflows  frequently  occur  in  small  bodies, 
and  it  will  probably  be  found  that  where  there  are  relatively  large  basic 
flows  a  portion  of  them  will  at  least  show  an  andesitic  habit. 

Differentiation  of  Lavas.— The  existence  at  Eureka  of  two  groups  of  lavas, 
differing  primarily  in  structure  and  the  chemical  nature  of  their  transition 
products,  has  been  clearly  demonstrated  and  evidence  has  been  advanced 
to  show  that  they  were  derived  from  a  still  earlier  molten  mass.  Processes 
of  differentiation  similar  to  those  by  which  the  molten  material  beneath  the 
surface  is  supposed  to  be  capable  of  breaking  up  into  rhyolite  and  basalt, 
are  sufficient  not  only  to  account  for  the  breaking  up  of  a  primordial 
mass  into  a  feldspathic  and  pyroxenic  magma,  but  also  to  account  for  the 
existence  of  partial  magmas  and  an  entire  series  of  transition  lavas  such  as 
found  at  Eureka.  The  first  products  of  such  a  molten  mass  would  naturally, 
but  not  necessarily,  be  a  lava  of  intermediate  composition,  such  as  are 
often  seen  as  the  earliest  eruptions  in  volcanic  centers.  The  first  eruptions 
at  Washoe  being  earlier  than  those  at  Eureka  were  consequently  more 
uniform  in  composition.  Differentiation  in  the  magma  had  taken  place 
only  to  a  limited  degree,  and  it  is  by  no  means  easy  to  distinguish 
homblende-audesite  from  pyroxene-andesite.  The  splitting  up  of  both 
the  feldspathic  and  pyroxenic  magmas,  the  former  into  hornblende- 
mica-andesite,  dacite,  and  rhyolite,  and  the  latter  into  pyroxene-andesite 
and  basalt,  has  already  been  described.  It  is  difficult  to  conceive  a  con- 
trolling physical  force  acting  upon  one  magma  which  could  not  under 
similar  conditions  of  heat  and  pressure  exert  the  same  influences  upon 
fractional  magmas,  the  differentiated  products  of  a  primordial  molten  ma». 


288  GEOLOGY  OF  THE  EUKEKA  DISTRICT. 

In  applying  this  hypothesis  of  differentiation  to  molten  masses  the 
question  naturally  arises,  What  would  have  resulted  at  Eureka  if  the  slow 
processes  of  differentiation  going  on  in  a  magma  before  final  crystallization 
had  either  terminated  earlier  or  progressed  still  further?  On  the  one 
hand,  supposing  a  separation  less  complete  than  that  at  Eureka,  a  stage  in 
the  development  would  be  reached  when  a  feldspathic  magma  would  form 
consisting  of  hornblende-mica-anclesite  or  dacite,  or  more  probably  both, 
followed  by  pyroxene-andesite  withoiit  the  interpolation  of  any  body  of 
rhyolite.  On  the  other  hand,  if  the  segregation  of  feldspathic  magma  had 
gone  on  more  completely  than  we  find  it  at  Eureka,  there  might  have  been 
formed  the  same  sequence  of  feldspathic  lavas,  only  with  a  much  larger 
extravasation  of  rhyolite,  in  turn  followed  by  basalt,  -without  the  inter- 
polation of  pyroxene-andesite.  Again,  the  earliest  rock  might  have  been 
hornblende-mica-andesite  of  the  feldspathic  magma,  succeeded  rapidly  by 
pyroxene-andesite.  If  this  series  of  lavas  had  been  followed  by  a  cessa- 
tion of  volcanic  energy  and  a  long  interval  of  rest,  and  then  by  a  renewal 
of  activity,  the  final  product,  after  a  still  further  separation  of  the  magma, 
would  result  in  the  extravasation  of  rhyolite  and  basalt.  This  latter 
sequence  of  lavas  gives  the  order  of  succession  so  frequently  met  with 
throughout  the  Great  Basin.  At  Eureka,  as  already  described,  no  long- 
time interval,  geologically  speaking,  is  recognized  between  the  andesites 
and  rhyolites,  while  the  dacites  and  rhyolites  frequently  present  the  appear- 
ance of  continuous  flows. 

In  considering  these  phenomena  it  is  important  to  bear  in  mind  the 
facts  so  frequently  observed  elsewhere  in  the  Great  Basin,  that  a  crystalline 
lava  derived  from  a  feldspathic  magma  of  intermediate  composition  is,  in 
many  instances,  as  shown  by  Richthofen  and  King,  followed  by  a  pyroxene 
lava,  and  the  latter  is  almost  invariably  of  intermediate  composition;  a  lava 
still  more  acid  by  one  correspondingly  basic,  and  the  extreme  acid  type  by 
the  extreme  basic  type.  A  rhyolite  may  be  followed  by  pyroxenic  lavas 
varying  in  composition,  but  the  writer  knows  no  instance  in  the  Great 
Basin  where  a  rhyolite  is  succeeded  by  a  more  basic  feldspathic  rock,  nor 
where  a  basalt  is  followed  by  a  less  basic  pyroxenic  lava. 


DIFFERENTIATION  OF  LAVAS.  289 

This  hypothesis  of  the  progressive  differentiation  by  molecular  changes 
in  a  fluid  or  a  molten  mass  under  varying  conditions  of  temperature  and 
pressure  is  offered  to  explain  the  variations  in  chemical  and  mineralogical 
composition  of  lavas.  It  i.s  offered  tentatively  and  with  much  hesitation,  the 
writer  knowing  the  many  difficulties  involved  in  the  problem.  It  is  based 
upon  and  is  in  accord  with  the  facts  seen  at  Eureka  and  confirmed  by 
observations  in  many  volcanic  areas  elsewhere.  It  at  least  has  the  merit  of 
accounting  for  nearly  all  variations  in  the  sequence  of  lavas  which  have 
from  time  to  time  been  noted  in  the  Great  Basin.  It  offers  a  rational 
explanation  for  the  recurrence  of  lavas  in  certain  localities  and  accounts  for 
their  absence  in  others.  The  pyroxene-andesite  furnishes  a  marked  instance 
of  such  a  recurrence.  Occurrences  of  lava  which  have  been  regarded  as 
exceptional  and  difficult  to  explain  by  any  general  law  are  now  seen  to 
fall  within  the  prescribed  limits  of  variation  as  laid  down  here.  Nothing 
seems  more  clear  than  that  there  are  certain  laws  determining  the  sequence 
of  flow  that  govern  the  extravasation  of  lavas  in  every  great  volcanic  cen- 
ter, notwithstanding  the  fact  that  we  may  still  be  a  long  way  from  the  cor- 
rect interpretation  in  all  its  details. 

Summary.— The  Eureka  District  presents  a  most  instructive  volcanic 
region  standing  quite  apart  from  all  other  centers  of  similar  eruption,  yet 
typical  in  the  nature  of  its  extravasated  material  of  many  localities  in  the 
Great  Basin. 

The  region  offers  no  direct  proof  of  the  age  of  volcanic  energy,  yet 
all  evidence  points  to  the  conclusion  that  the  eruptions  belong  to  the  Ter- 
tiary period  and  for  the  most  part  to  the  Pliocene  epoch.  They  may  have 
extended  well  on  into  Quaternary  time,  although  there  is  no  reason  to  sup- 
pose that  eruptions  took  place  within  historic  periods. 

As  regards  their  mode  of  occurrence  the  principal  eruptions  may  be 
classed  under  four  heads :  First,  they  broke  out  through  profound  fissures 
along  the  three  great  meridional  lines  of  displacement,  the  Hoosac,  Pinto, 
and  Rescue  faults,  and  to  some  extent  along  the  lesser  parallel  faults; 
second,  following  the  lines  of  orographic  fracture,  they  border  and  almost 
completely  encircle  the  large  uplifted  masses  of  sedimentary  strata  like  the 
MON  xx 19 


290  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

Silverado  and  County  Peak  block  and  the  depressed  Carboniferous  block 
between  the  Hoosac  and  Pinto  faults  ;  third,  they  occur  in  numerous  dikes 
penetrating  the  limestones ;  fourth,  they  occur  in  one  or  two  relatively 
large  bodies,  notably  Richmond  Mountain  and  Pinto  Peak,  along  lines  of 
displacement  already  mentioned. 

All  the  lavas  may  be  classed  under  the  heads :  hornblende-andesite, 
hornblende-mica-andesite,  dacite,  rhyolite,  pyroxene-andesite,  and  basalt. 
They  pass  by  insensible  gradations  from  one  to  the  other.  All  division  lines 
are  more  or  less  arbitrary ;  they  are  necessary  for  the  purposes  of  classifica- 
tion, although  they  may  not  exist  in  nature. 

Field  observations  clearly  show  that  the  order  of  succession  of  these 
natural  groups  into  which  the  lavas  have  been  divided  was  as  follows  :  First, 
that  the  homblende-audesite  was  the  earliest  of  all  the  erupted  material ; 
second,  that  the  hornblende-mica-andesite  followed  the  hornblende-ande- 
site ;  third,  that  the  dacite  followed  the  hornblende-mica-andesite ;  fourth, 
that  the  rhyolite  closely  followed  the  dacite;  fifth,  that  the  pyroxene-ande- 
site succeeded  the  rhyolite ;  sixth,  that  the  basalt  was  the  most  recent  of  all 
these  volcanic  products. 

In  chemical  composition  this  entire  series  of  lavas  shows  a  range  in 
silica  amounting  to  about  25  per  cent,  a  range  which  is  quite  as  wide  as  is 
usually  found  in  most  centers  of  eruption  even  where  the  volume  of  lavas 
thrown  out  is  vastly  greater  and  the  duration  of  volcanic  energy  far  longer. 
Analyses  show  endless  transition  products  between  the  extreme  basic  and 
acidic  lavas,  with  a  tendency  of  the  alkalies  and  silica  to  accumulate  at  the 
acidic  end  and  the  material  forming  the  ferro-magnesian  minerals  at  the 
basic  end. 

It  is  maintained  in  this  work  that  all  the  varied  products  of  eruption 
are  derived  from  a  common  source,  a  homogeneous  molten  mass.  Under  a 
process  of  differentiation  this  earlier  mass  split  up  into  two  magmas,  desig- 
nated as  a  feldspathic  and  a  pyroxenic  magma.  The  lavas  at  Eureka  are  the 
result  of  the  same  process  of  differentiation  derived  from  one  or  the  other 
of  these  magmas.  Beginning  with  hornbende-andesite,  the  earliest  lava,  the 
feldspathic  magma  became  more  siliceous  until  the  close  of  rhyolitie  erup- 
tions. The  rhyolite  was  followed  by  pyroxene-andesite  and  the  eruptions 


SUMMAEY.  291 

became  more  and  more  basic  until  the  close  of  the  volcanic  period.  The 
feldspathic  and  pyroxenic  lavas  do  not  approach  each  other  in  their  tenure 
of  silica  within  2-25  per  cent.  In  chemical  composition  the  earliest  erup- 
tions of  both  magmas  resemble  each  other,  but  from  this  common  ground 
they  differentiate  steadily  until  the  feldspathic  reaches  the  extreme  acidic, 
and  the  pyroxenic  the  extreme  basic  end  of  their  respective  series.  The 
extreme  products  of  differentiation  in  any  volcanic  center  in  the  Great 
Basin  are  rhyolite  and  basalt. 


CHAPTER    IX. 
ORE  DEPOSITS. 

Geological  History.— It  is  not  the  iuteiitioii  to  enter  into  a  detailed  descrip- 
tion of  the  various  ore  deposits  of  this  region  or  of  their  mode  of  occur- 
rence. An  excellent  monograph  upon  the  mines  and  ores  of  Ruby  Hill 
has  been  published  by  Mr.  J.  S.  Curtis,1  in  which  he  gives  in  much  detail 
the  results  of  his  studies  of  the  silver-lead  deposits  of  the  Richmond  and 
Eureka  mines. 

This  report,  however,  would  be  incomplete  if  the  writer,  after  devoting 
much  time  to  an  investigation  of  the  structural  features  of  the  Eureka 
Mountains,  constantly  keeping  in  mind  the  relationship  between  the  ore 
bodies  and  the  sedimentary  and  igneous  rocks,  should  fail  to  state  his  con- 
clusions as  to  the  geological  position  of  the  ores,  their  age,  and  origin. 
Moreover,  as  many  geologists  do  not  care  for  the  details  of  mining  develop- 
ments, but  feel  a  keen  interest  in  all  questions  relating  to  mineral,  deposi- 
tion, it  seems  desirable  to  state  here,  for  the  use  of  the  general  reader,  such 
facts  as  bear  directly  upon  the  geological  occurrence  of  the  Eureka  ore 
bodies. 

It  has  been  demonstrated  beyond  all  question,  from  the  facts  presented 
in  the  preceding  chapters,  that  the  Eureka  Mountains  are  formed  of 
orographic  blocks  of  Paleozoic  strata  made  up  of  quartzites,  limestones, 
and  shales.  These  blocks,  strongly  contrasted  by  their  orographic  struc- 
ture, are  separated  from  each  other  by  profound  north  and  south  faults. 
Along  the  lines  of  these  displacements  east  of  Prospect  Ridge  enormous 
masses  of  igneous  rocks  have  been  poured  out,  which  have  tended  still 
more  sharply  to  intensify  the  lines  of  demarcation  between  the  individual 
blocks.  The  entire  thickness  of  Paleozoic  sediments  can  not  be  far  from 


1  Silver-lead  deposits  of  Eureka,  Nevada.     Mon.  U.  S.  Geol.  Survey,  VII,  Washington,  1884. 
292 


RHYOLITES  AND  OKE  DEPOSITS.  293 

30,000  feet.  Between  the  close  of  the  Carboniferous  and  the  close  of  the 
Jurassic  period  dynamic  action  folded  and  faulted  the  strata,  producing  the 
present  complex  structure  and  outlining  the  configuration  of  the  mountains 
much  as  they  are  found  to  exist  to-day  except  such  changes  as  have 
been  produced  by  denudation.  Soon  after  the  coming  in  of  Tertiary 
time  the  volcanic  period  began  in  the  Great  Basin,  and  probably  not 
long  after  it  volcanic  energy  manifested  itself  in  the  Eureka  Mountains. 
Evidence  seems  to  show  that  the  profound  displacements  were  augmented 
by  intrusive  rocks,  and  in  many  instances  fissures  were  formed  along  the 
fault  planes.  Accompanying  the  fissuring  of  these  faults  by  volcanic  lavas 
was  the  forming  of  lateral  and  oblique  secondary  faults,  cross  fissures,  and 
fractures,  complicating  the  already  disturbed  sedimentary  beds. 

After  the  outbursts  of  andesites  and  rhyolites,  and  possibly  in  part 
subsequent  to  the  basalts,  the  deposition  of  the  ores  took  place.  The  basalt 
is  known  to  follow  the  rhyolite.  As  regards  the  relative  age  of  the  ores  and 
basalt,  there  is  no  direct  evidence  other  than  that  in  the  region  of  ore 
bodies  the  audesites  and  rhyolites  show  the  action  of  steam  and  solfataric 
agents,  whereas  the  basalts  are  for  the  most  part  comparatively  fresh  and 
unaltered.  In  a  number  of  instances  it  is  clearly  evident  that  the  ores 
followed  the  rhyolite  intrusions,  the  former  being  found  to  lie  wholly  undis- 
turbed upon  the  latter  rock.  It  is  true  that  over  the  greater  part  of  the 
region  the  ores  do  not  come  in  direct  contact  with  the  rhyolites,  but,  on  the 
other  hand,  all  evidences  of  pre-rhyolite  ore  deposits  are  wholly  wanting, 
and  it  is  hardly  conceivable  that  there  could  have  been  such  deposits  with- 
out some  evidence  of  movement  at  a  time  when  the  region  was  undergoing 
strain  and  dislocation  on  all  sides.  Furthermore,  nowhere,  so  far  as  known, 
does  the  network  of  dikes  on  Prospect  Ridge  cut  any  earlier  ore  body. 

Since  the  intrusion  of  the  innumerable  rhyolite  dikes  there  is  no 
evidence  of  any  orographic  movement  of  sufficient  intensity  to  disturb  or 
dislocate  them  by  faulting  of  the  strata,  and  the  same  may  be  said  of 
the  ore  deposits.  This  gives  both  to  the  dikes  and  ores  a  comparatively 
recent  origin  in  the  geological  history  of  the  region.  As  regards  the  ores 
it  should  be  stated  that  they  have  undergone  alteration  and  oxidation  since 
their  deposition,  and,  as  much  of  the  loose,  friable  material  occurs  in  lime- 


294  GEOLOGY  OF  THE  EUKEKA  DISTRICT. 

stone  chambers  and  cavities,  it  is  quite  likely  to  have  undergone  some 
movement  by  earthquake  shocks  during  Quaternary  time,  but  this  is  quite 
another  matter  from  profound  orographic  displacement  of  beds.  To-day 
there  is  absolutely  nothing  positively  known  as  to  the  source  of  the  rhyolite 
material  nor  the  deep-seated  centers  from  which  it  originated,  and  this 
is  equally  true  as  to  the  source  of  the  heavy  metals.  With  our  lack  of 
knowledge  on  these  matters  it  seems  out  of  place  to  speculate  in  the  present 
volume  as  to  their  ultimate  source.  Probably  no  geologists,  however, 
would  question  the  statement  that  the  volcanic  products  came  from  below. 

The  writer,  after  a  careful  study  of  the  facts  observed  at  Ruby  Hill 
and  Prospect  Mountain,  as  well  as  of  the  entire  Eureka  region,  is  forced  to 
the  conclusion  that  there  exists  the  closest  relationship  between  the  rhyo- 
lites  and  the  formation  of  the  ore  deposits,  although  they  have  been 
observed  in  actual  contact  in  only  one  or  two  localities  in  the  larger  mines. 
In  almost  all  cases  where  mineral  deposits  are  found,  rhyolite  intrusions  are 
known  to  penetrate  the  limestone  in  close  proximity  to  the  ores,  and  it  is 
presumable  that  in  many  instances  the  presence  of  such  ore  bodies  might 
be  detected  without  the  discovery  of  any  intrusive  rock.  No  theory  of  the 
ore  deposits  seems  applicable  to  this  region  that  does  not  carry  with  it  the 
fundamental  proposition  that  the  ores  came  from  below,  as  the  result  of  sol- 
fataric  action  which  accompanied  volcanic  energy.  Evidence  shows  that 
the  centers  of  greatest  deposition  of  ores  were  not  the  same  as  those  of 
greatest  eruptive  energy,  but  that  the  latter  are  associated  with  the  secondary 
dikes  and  offshoots  rather  than  with  the  great  lines  of  volcanic  outbursts. 
Solfataric  action  may  have  continued  and  probably  did  continue  for  a  long 
period  after  the  rhyolite  eruptions  had  altogether  ceased,  during  which 
metallic  sulphides  filled  certain  preexisting  fissures,  cracks,  chambers,  and 
crevices  in  the  limestone.  After  the  deposition  of  the  sulphides  came  the 
period  of  oxidation  which,  so  far  as  can  be  told,  lasted  throughout  the 
greater  part  of  Quaternary  time. 

Ores  of  the  Cambrian.— In  regard  to  the  distribution  and  geological  position 
of  the  ores  in  the  Paleozoic  strata  all  evidence  shows  that  although  the 
most  remunerative  mines  and  those  explored  to  a  great  depth  occupy  some- 
what restricted  limits,  ores  of  similar  mode  of  occurrence  and  composition 


ORES  OF  THE  CAMBKIAN.  295 

as  those  so  successfully  worked  on  Ruby  Hill,  are  found  throughout  a  wide 
vertical  range  of  sedimentary  beds.  No  ore  deposits  are  known  below  the 
contact  between  the  Prospect  Mountain  quartzite  and  the  overlying  lime- 
stones upon  Ruby  Hill.  As  will  be  shown  later  these  limestones  on  Ruby 
Hill  carry  deposits  of  ore  throughout  their  entire  thickness  from  the  quartz- 
ite to  the  overlying  Secret  Canyon  shale. 

Along  the  slopes  of  Prospect  Mountain  from  Mineral  Hill  southward 
to  Surprise  Peak,  the  crushed  and  brecciated  limestones  have  undergone 
considerable  local  disturbance  and  are  honeycombed  throughout  by  fissures, 
seams,  and  irregular  crevices  of  various  width  and  length.  Many  of  these 
openings  lie  parallel  with  the  stratification ;  others  cut  across  the  beds,  occur- 
ring in  the  limestone  anywhere  between  the  quartzite  and  shale  without  any 
recognized  order.  Oxidized  ore  bodies  occupy  these  openings,  many  of 
them  being  connected  by  narrow  channels  and  seams  more  or  less  filled 
with  mineral  matter.  The  William.sburg  mine  on  the  west  side  is  a  good 
example  of  the  ore  found  deposited  in  characteristic  chambers,  while  on  the 
east  side  at  the  extreme  southern  end  of  the  ridge  the  Geddes  and  Bertrand 
mine  appears  to  be  a  well  defined  north  and  south  fissure  carrying  much 
rich  ore.  Among  others  of  the  larger  bodies  of  ore  may  be  mentioned  those 
of  the  Silver  Connor  and  Banner  mines,  the  latter  a  good  example  of  a  fissure 
which  occurs  on  the  summit  of  the  ridge.  In  but  few  of  these  ore  bodies, 
at  least  on  the  surface,  have  any  rhyolites  been  recognized.  A  marked 
instance,  however,  may  be  seen  in  the  case  of  the  Geddes  and  Bertrand 
mine,  where  a  powerful  east  and  west  dike  cuts  the  limestone  and  overlying 
shale  in  close  proximity  to  the  north  and  south  ore  channel. 

Nowhere  along  the  grand  exposures  of  Secret  Canyon  shales  have  the 
ores  penetrated  to  the  surface,  the  pliable,  argillaceous  clays  flexing  and 
folding  instead  of  fissuring,  and  everywhere  serving  as  an  impervious 
barrier  to  the  ascending  currents.  Fine  examples  of  dike  cutting  are 
shown  near  the  Greddes  and  Bertrand  Mine  and  again  on  the  summit  of  the 
watershed  between  New  York  Canyon  and  Secret  Canyon  shales,  but  at 
the  latter  locality,  so  far  as  known,  wholly  unaccompanied  by  important 
mineral  matter. 

The    beds  of  the  Hamburg  limestone  are  similar  in  their  structural 


296  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

features  to  those  of  Prospect  Mountain,  the  resemblance  holding  equally 
good  for  the  ore  bodies.  On  Adams  Hill  the  Price  and  Davies  mine  lies  in 
this  formation  in  close  contact  with  the  Secret  Canyon  shales,  whereas  the 
Wide  Wide  West  occurs  near  the  summit  just  below  the  Hamburg  shales. 
Other  localities  where  more  or  less  work  has  been  done  were  sufficient  to 
indicate  the  existence  of  mineral  deposits  across  the  intervening  belt  of 
limestone  from  one  shale  belt  to  the  other.  Along  Hamburg  Ridge  the 
limestones  are  not  so  much  disturbed  as  on  the  steeper  mountain  slopes,  and 
fissures  and  seams  of  ore  are  by' no  means  as  common,  but  on  the  other  hand 
mines  like  the  Duuderberg  and  Hamburg  have  produced  large  bodies  of  ore, 
second  in  quantity  to  none  in  the  district  outside  of  Ruby  Hill,  and  these 
stand  in  the  closest  connection  with  intrusive  masses  of  rhyolite.  Dikes 
and  irregular  shaped  bosses  of  rhyolite  along  the  summit  of  Hamburg  Ridge 
indicate  a  network  of  eruptive  rocks  between  the  two  great  shale  belts. 
Like  the  underlying  Secret  Canyon  shale  horizon,  the  Hamburg  shales, 
although  of  much  less  thickness,  are  impervious  to  ascending  mineral  cur- 
rents, and  neither  along  the  front  of  the  mountain  or  north  of  Adams  Hill 
is  there  the  slightest  evidence  of  ore  bodies  penetrating  it. 

Ores  of  the  Silurian.— Coming  to  the  Pogonip  horizon,  ore  bodies  occur  all 
the  way  from  the  north  end  of  Adams  Hill  southward  to  Roundtop 
Mountain,  at  the  extreme  southern  end  of  the  region,  with,  however,  con- 
siderable intervals  where  none  have  been  exposed  near  the  surface.  Numer- 
ous mining  claims  have  from  time  to  time  been  recorded,  but  most  of  the 
ground  proved  unprofitable  and  unproductive.  On  the  other  hand,  such 
mines  as  the  Bullwhacker  and  Williamsburg,  northwest  of  the  town  of 
Eureka,  and  the  Page  and  Corwin,  southwest  of  Pinto  Peak,  have  yielded 
large  quantities  of  mineral  matter  and  may  be  said  to  exhibit  well  its 
mode  of  occurrence  in  the  limestone  of  this  horizon.  In  the  Williams- 
burg  Mine  a  well  defined  quartz-porphyry  dike  penetrates  the  limestone, 
and  dikes  of  similar  rock  come  to  the  surface  near  the  Bullwhacker.  The 
Page  and  Corwin  was  not  being  worked  at  the  time  of  the  writer's  visit, 
and  it  is  impossible  to  say  whether  any  intrusive  dikes  have  broken 
through  the  strata  in  close  connection  with  the  ore,  but  the  limestones  are 


ORES  OF  THE  DEVONIAN.  297 

4 

much  disturbed  and  faulted  and  rhyolite  has  reached  the  surface  only  a 
short  distance  from  the  mining  property. 

In  the  Eureka  quartzite  the  only  instances  known  of  mineral  deposi- 
tion are  those  found  on  Hoosac  Mountain,  a  description  of  which  is  given 
elsewhere.  They  have  been  worked  extensively  and  have  yielded  consider- 
able ore.  Here  they  are  intimately  associated  with  intrusive  dikes  of  both 
andesite  and  rhyolite  offshoots  from  the  great  bodies  which  forced  their 
way  upward  along  the  Hoosac  fault. 

Throughout  the  Eureka  Mountains  the  Lone  Mountain  horizon  has 
here  and  there  shown  evidences  of  mineral  deposits  when  found  in  the 
neighborhood  of  rhyolite  outbursts,  but  over  the  greater  part  of  the  area 
they  exhibit  no  surface  signs  of  ore-bearing  material.  An  interesting 
example  of  ore  in  the  Lone  Mountain  horizon  may  be  found  at  the 
Seventy-six  mine,  in  hard,  flinty  limestone  on  the  northwest  side  of 
McCoy's  Ridge.  While  it  can  not  be  looked  upon  as  remunerative  property, 
from  the  point  of  view  of  the  present  description  it  serves  as  an  instructive 
link  in  the  chain  of  facts  bearing  upon  the  geological  position  of  the 
Eureka  ore  bodies.  This  is  the  only  body  of  Lone  Mountain  limestone 
lying  in  close  proximity  to  the  Hoosac  fault,  and,  in  consequence,  partially 
explains  the  occurrence  of  ore. 

Ores  of  the  Devonian.— Passing  upward,  without  any  intervening  lithological 
break,  the  Nevada  limestones  are  in  like  manner  frequently  found  to  carry 
oxidized,  argentiferous  lead  ores  in  fissures  and  crevices  in  the  regions  of 
profound  faults.  It  by  no  means  follows  that  rhyolites  necessarily  accom- 
pany the  ore  at  the  surface,  and  still  less  that  the  latter  occurs  wherever 
rhyolite  penetrates  the  Nevada  limestone  through  fissure  planes.  Instances 
may  be  cited  in  the  case  of  the  Reese  and  Berry  mine,  just  north  of  the 
canyon  of  the  same  name,  and  again  on  the  summit  of  Newark  Mountain, 
both  localities  indicating  disturbances  of  strata  without  any  .assignable 
cause  on  the  surface.  The  dislocation  of  beds  may  be  due  to  intrusive 
rocks  which  failed  to  penetrate  the  surface,  the  fissuring  being  filled  by 
mineral  matter. 

Along  Rescue  Canyon,  where  there  is  such  a  continuous  and  powerful 
mass  of  rhyolite  under  geological  conditions  similar  in  many  respects  to 


298  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

those  observed  along  the  Hoosac  fault,  mineral  deposits  are  found  identical 
in  mode  of  occurrence  with  those  found  on  Prospect  Mountain,  although 
less  productive.  A  line  of  ore  deposits  follows  Rescue  Canyon  on  the  east 
side  in  the  highly  inclined  strata  of  Century  Ridge.  The  Rescue  mine  is 
the  most  important  property,  exploration  having  developed  several  small, 
but  rich,  chambers  of  ore  in  following  down  the  shaft  between  400  and  500 
feet  below  the  surface.  Other  mines  on  Century  Ridge  are  the  Queen  and 
Maryland,  both  of  which  resemble  the  Rescue  mine. 

In  the  Alhambra  Hills  a  shaft  has  been  sunk  in  the  Fairplay  mine,  85 
feet  in  depth,  following  down  a  clay  seam  between  well  denned  walls  of 
limestone.  It  carries  a  good  deal  of  galena.  The  White  Rose  mine  closely 
resembles  the  Fairplay  and  lies  in  nearly  the  same  geological  horizon. 

Crossing  to  the  Mahogany  Hills,  on  the  opposite  or  west  side  of  the 
Eureka  Mountains,  we  find  mining  properties  on  the  southeast  side  of 
Brush  Peak  at  localities  designated  as  the  Mountain  Boy  and  Kentuck  mines. 
They  show  that  mineral  matter  was  deposited  under  geological  conditions 
similar  to  those  found  elsewhere.  Again,  at  the  head  of  Browns  Canyon 
there  is  a  very  decided  break  in  the  limestone,  accompanied  by  a  sharp 
anticlinal  fold,  along  the  axis  of  which  occurs  an  outflow  of  rhyolite  pre- 
senting geological  conditions  that  might  readily  lead  one  to  look  for  ore. 
Indications  of  mineral  deposits  were  found  at  the  surface  sufficient  to  war- 
rant mining  exploration,  and  an  ore  channel  followed  for  considerable 
distance  into  the  limestone.  A  study  of  the  geological  position  of  these 
different  ore  bodies  makes  it  clear  that  they  occur  throughout  the  Nevada 
limestone,  being  found  near  the  base  of  the  epoch  and  again  not  far  below 
the  summit.  With  the  coming  in  of  the  White  Pine  shales  all  the  charac- 
teristic oxidized  and  unoxidized  ores  of  the  district  cease,  and  they  fail  to 
reappear  in  any  of  the  higher  geological  horizons. 

NO  ores  in  the  Carboniferous.— Nowhere  within  the  district  have  ores  been 
recognized  in  any  of  the  grand  divisions  of  Carboniferous  time.  In  the 
Diamond  Range  northward  and  westward  of  Newark  Mountain  the  strata 
seem  to  be  entirely  free  from  mineral  matter.  It  is  possible  that  mining  claims 
may  have  been  recorded  along  some  superficial  outcrop  or  some  segregation 
of  mineral  matter,  but  these  are  so  obscure  and  unpromising  and  usually 


KANGE  OF  ORE  DEPOSITS.  299 

without  any  indication  of  the  precious  metals  that  they  may  be  wholly 
discarded.  The  same  may  be  said  of  the  entire  area  of  the  Carboniferous 
block  lying  between  the  Hoosac  and  Pinto  faults.  It  is  somewhat  remark- 
able that  in  this  latter  block,  which  lies  in  the  very  center  of  volcanic 
action,  no  mineral  occurrences  of  any  importance  are  known.  Along  the 
two  great  meridional  faults  enormous  masses  of  igneous  rocks  have  been 
poured  out,  notwithstanding  which  no  ore  deposits  have  been  reported 
either  on  the  east  side  of  the  Hoosac  fault  or  on  the  west  side  of  the 
Pinto  fault. 

Geological  Range  of  Ore  Deposits.— It  will  be  seen  from  these  facts  that  the 
ore  deposits  of  Eureka  are  found  throughout  a  wide  vertical  range,  extend- 
ing from  the  base  of  the  Prospect  Mountain  limestone  to  the  summit  of  the 
Nevada  limestone,  occurring  in  every  grand  division  of  the  Cambrian, 
Silurian,  and  Devonian  periods,  with  the  exception  of  the  two  great  shale 
belts — the  Secret  Canyon  and  Hamburg  shales.  From  the  base  of  the 
Prospect  Mountain  limestone  to  the  top  of  the  Hamburg  shale  it  is  esti- 
mated that  there  are  6,200  feet  of  strata;  the  Siluiian  rocks  measure  5,000 
feet  and  the  Nevada  limestone  of  the  Devonian  6,000  feet.  This  gives 
from  the  base  to  the  summit  of  the  included  strata  over  17,000  feet  of 
sedimentary  rocks,  through  which  argentiferous  lead  ores  have  been 
deposited  on  a  sufficiently  extensive  scale  to  encourage  more  or  less 
expensive  outlays  for  mining  exploration. 

From  the  rapid  review  of  these  facts  it  is  evident  that  within  the  area 
of  the  Eureka  District  the  ores  are  by  no  means  restricted  to  any  definite 
geological  horizons  and  have  been  deposited  in  siliceous  as  well  as  calcareous 
strata.  Notwithstanding  that  the  ore  .  bodies  occur  through  a  great  thick- 
ness of  rock,  it  still  remains  true  that  the  greater  part  of  the  mineral  depos- 
its and  probably  all  those  which  have  proved  remunerative  to  the  investor, 
lie  within  restricted  limits.  The  most  productive  mines,  those  carrying 
the  largest  and  richest  bodies  of  ore,  are  found  in  Cambrian  strata.  This 
is  owing  to  orographic  and  structural  conditions  rather  than  to  the 
geological  age  of  strata  or  the  chemical  nature  of  sediments.  A  study  of 
the  structural  features  of  the  mountains  together  with  the  mode  of  occur- 
rence of  the  rhyolite  eruptions  shows  that  the  age  of  the  rock  has  but  little, 


300  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

if  anything,  to  do  with  the  occurrence  of  the  deposits.  They  depend  more 
upon  the  fissuring  and  fracturing  of  the  mountain  uplifts  and  the  relations 
of  the  accompanying  faults  to  the  outbursts  of  rhyolite. 

A  study  of  the  mountain  blocks  and  distribution  of  ores  brings  out  the 
fact  that  what  has  been  designated  the  Prospect  Mountain  uplift,  lying 
between  the  Hoosac  fault  on  the  east  side  and  the  Sierra  and  Spring  Valley 
faults  on  the  west,  embraces  pretty  much  all  the  valuable  mineral  deposits 
which  have  as  yet  been  successfully  developed.  Within  the  limits  of  these 
lines  of  faulting  are  embraced  all  the  mining  properties  extending  from 
Adams  Hill  southward  to  Surprise  Peak,  including  those  on  the  west  side 
of  Prospect  Mountain,  together  with  the  Dugout  mine  at  the  southwest  base 
of  the  peak  on  the  west  side  of  the  anticlinal  fold.  In  preceding  chapters 
the  structural  features  of  Prospect  Mountain  Ridge  and  relations  between 
the  sedimentary  beds  and  intrusive  dikes  have  been  described  with  some 
detail.  As  has  already  been  shown,  the  strata  between  these  faults  belong 
to  the  Cambrian  and  Silurian  periods  up  to  and  including  the  lower  portion 
of  the  Lone  Mountain  horizon  exposed  on  the  north  side  of  McCoy's  Ridge. 
The  principal  overflows  of  rhyolite  have  been  along  the  line  of  the  Hoosac 
fault,  the  two  most  powerful  centers  of  extravasation  being  located  at  Pinto 
Peak  and  Purple  Mountain.  Purple  Mountain  lies  in  the  angle  between  the 
Hoosac  and  Ruby  Hill  faults,  and  it  is  along  this  latter  fault  that  rhyo- 
lites  come  to  the  surface  all  the  way  from  New  York  Canyon  to  the  Jack- 
son fault,  thence  crossing  the  latter  fault,  fill  the  fault-fissure  for  a  consid- 
erable distance  along  the  north  slope  of  Ruby  Hill,  but  without  building 
up  any  accumulation  of  rhyolite  on  the  surface. 

While  it  can  not  be  absolutely  demonstrated,  all  evidence  bears  out  the 
assumption  that  the  dikes  penetrating  Prospect  Mountain  Ridge  have  a 
deep-seated  connection  with  the  source  of  the  rhyolite  material  which  has 
furnished  the  surface  outflows  all  along  the  line  of  faulting.  It  can  hardly 
be  doubted  that  both  forms  of  the  same  eruptive  rock  mass  have  had  an 
identical  deep-seated  origin.  It  should  be  also  borne  in  mind  that  it  is  only 
in  exceptional  instances  that  dikes  and  off-shoots  from  any  parent  body  of 
lava  can  be  traced  to  their  source  step  by  step  in  the  field  without  any 
break  in  continuity. 


EUBY  HILL  DEPOSITS.  301 

Ruby  Hill  Ore  Deposits.— Mr.  J.  S.  Curtis,  in  his  elaborate  monograph  upon 
the  ore  deposits  of  Ruby  Hill,  has  embodied  the  results  of  much  careful 
investigation  of  the  underground  exploitations  of  the  mines.  His  work  is 
accompanied  by  numerous  vertical  and  cross  sections,  compiled  from  the 
original  mine  surveys,  indicating  the  positions  of  the  different  ore  bodies 
and  their  mutual  relations.  It  is  unnecessary,  therefore,  to  enter  into  the 
details  of  the  economic  geology,  and  only  such  facts  will  be  given  as  will 
enable  the  reader  to  form  a  correct  conception  of  the  geological  position 
and  mode  of  occurrence  of  the  ore  bodies,  not  only  upon  Ruby  Hill  but 
those  found  throughout  the  district.  Ruby  Hill,  from  a  geological  point  of 
view,  may  be  taken  as  typical  of  the  deposits  in  what  has  been  designated 
as  the  Prospect  Mountain  uplift.  For  the  details  of  the  mines,  and  their 
extensive  underground  workings  the  reader  is  referred  to  Mr.  Curtis's  report. 
In  chapter  v,  of  this  report,  upon  the  descriptive  geology,  there  will  be 
found  an  account  of  the  geological  structure  of  Ruby  Hill  and  Adams 
Hill. 

By  reference  to  the  accompanying  map  (PL  i,)  and  section  (Fig.  3,  PL 
ii,)  it  will  be  seen  that  Ruby  Hill  is  made  up  of  the  tliree  underlying  forma- 
tions of  the  Cambrian.  They  possess  a  fairly  uniform  dip,  although  pre- 
senting occasional  abrupt  changes  due  to  faults  and  fractures.  This  dip 
along  the  surface  may  be  taken  at  40°,  and  in  the  lowest  workings  of  the 
Richmond  mine,  which  have  reached  a  depth  of  over  1,200  feet,  this  angle 
of  inclination  is  still  maintained.  At  the  surface  the  line  of  contact  between 
the  quartzite  and  limestone  is  easily  made  out  all  the  way  from  the  Jackson 
fault  to  the  west  base  of  Ruby  Hill.  Near  this  latter  fault  the  contact  is 
first  observed  just  west  of  the  American  shaft  and  the  Jackson  mine.  It 
crosses  the  summit  of  the  spur  on  which  the  Phoenix  mine  is  situated  and 
follows  along  the  southern  slope  of  Ruby  Hill,  the  quartzite  at  one  point 
rising  to  within  160  feet  of  the  summit.  In  the  underground  workings  flu- 
plane  of  contact  between  the  quartzite  and  limestone  has  been  exposed  in 
all  the  mines  and  at  very  many  of  the  levels,  the  latter  frequently  running 
along  the  contact  of  the  two  formations  and  occasionally  cutting  the 
quartzite  where  it  projects  to  the  north  beyond  the  course  of  the  drifts. 
Numerous  cross-cuts  have  also  been  run  into  the  underlying  rock. 


302  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

These  workings,  although  they  do  not  offer  a  continuous  exposure,  are 
sufficient  to  give  the  course  of  the  quartzite  all  the  way  from  the  Jackson 
to  the  Albion.  Beginning  with  the  Jackson  mine,  the  plane  of  contact  has 
a  course  a  little  east  of  north,  gradually  turning  more  and  more  to  the  west 
until  at  the  Albion  it  curves  slightly  south  of  west.  In  the  lower  levels  of 
the  mines  this  contact  plane,  as  mapped  by  the  underground  surveys,  pre- 
sents roughly  a  concave  outline  curving  outward  toward  the  north  or  away 
from  the  granite  mass  of  Mineral  Hill.  This  curve,  however,  is  by  no  means 
symmetrical,  the  tendency  of  the  quartzite  in  making  so  sharp  a  bend 
being  to  break  abruptly  and  irregularly  and  for  short  distances  to  be  forced 
outward,  assuming  directions  quite  at  variance  with  the  general  course ; 
the  dip  frequently  changing  with  the  strike.  This  tendency  of  the  quartz- 
ite in  fracturing  to  be  forced  outward  beyond  the  line  of  the  curve  is  well 
shown  just  west  of  what  is  known  as  the  compromise  line  between  the 
Eureka  and  Richmond  mines.  It  may  be  seen  all  the  way  from  the  sur- 
face down  to  the  ninth  level  of  the  Richmond.  Mr.  Curtis  has  carefully 
mapped  the  underground  contacts,  not  only  between  the  quartzite  and  lime- 
stone but  for  all  three  formations.  By  reference  to  his  map1  both  the  con- 
cave outline  of  the  beds  and  the  irregularities  of  strike  and  dip  may  be 
seen  at  a  glance. 

The  overlying  limestone  and  shale  conform  in  their  general  outline 
with  the  quartzite,  the  shale,  however,  exhibiting  a  decided  tendency,  as  is 
usually  the  case  with  argillaceous  strata,  to  bend  and  fold  rather  than  to 
break  abruptly. 

Across  the  limestone  on  the  east  slope  of  Ruby  Hill  runs  a  profound 
fault  which,  on  account  of  its  bearing  upon  the  geology  of  the  region,  has 
been  designated  the  Ruby  Hill  fault.  It  is  a  continuation  of  a  line  of 
faulting  coming  up  from  the  southwest.  From  New  York  Canyon,  where 
the  Ruby  Hill  fault  leaves  the  Hoosac  fault,  to  its  intersection  with  the 
Jackson  fault  the  nearly  straight  course  is  easily  followed  by  a  chain  of 
rhyolite  outbursts  as  well  as  by  the  conformity  of  strata  Where  it 
crosses  the  Jackson  fault  its  direction  is  somewhat  disturbed  and  is  not  so 
readily  made  out,  but  near  the  American  shaft  it  reappears,  with  a  course  a 

'Op.  Cit.,  PI.  ni. 


RUBY  HILL  FAULT. 

few  degrees  west  of  north,  passing  just  west  of  the  Jackson  shaft.  From 
this  point  westward  it  is  difficult  to  follow  the  Ruby  Hill  fault  on  the 
surface,  as  it  lies  wholly  in  limestone  more  or  less  concealed  by  soil  and 
de"bris,  and  the  rhyolite  which  to  the  southeast  of  the  Jackson  fault 
materially  aids  in  tracing  the  displacement  nowhere  comes  to  the  surface 
after  crossing  the  latter  fault.  At  the  time  the  accompanying  map  was 
printed  the  line  of  the  Ruby  Hill  fault  had  not  been  followed  west  of  the 
Jackson  mine,  but  since  then  Mr.  Curtis  has  traced  it  through  the  under- 
ground workings  of  all  the  mines  as  far  as  the  extreme  limit  of  exploration 
in  the  Albion. 

According  to  the  investigations  of  Mr.  Curtis  the  fault  after  leaving  the 
Phoenix  mine  runs  in  a  nearly  northwest  direction,  agreeing  closely  with  its 
course  on  the  east  side  of  the  Jackson  fault.  It  passes  just  to  the  north- 
east of  the  KK  shaft  and  southwest  of  the  Richmond  office.  It  persistently 
cuts  all  formations,  quartzites,  limestones,  and  shales  alike,  scarcely  devia- 
ting from  a  straight  line  and  apparently  uninfluenced  by  the  physical 
conditions  of  the  rock.  In  like  manner  the  fractures  and  displacements 
produced  by  the  earlier  orographic  changes  whicli  elevated  the  region  have 
exerted  but  little  influence  on  the  course  of  the  Ruby  Hill  fault.  A  study 
of  the  disturbances  and  dislocations  of  the  strata  point  to  the  conclusion 
that  this  fault,  with  its  accompanying  fissure,  was  the  last  dynamic  move- 
ment in  the  history  of  Ruby  Hill.  Wherever  underground  explorations 
admitted  of  observation  the  average  dip  of  the  fissure  plane  was  found  to 
be  about  70°  to  the  northeast.  Southeast  of  the  Jackson  fault  the  width  of 
the  fissure  and  the  dip  of  its  plane  are  unknown. 

Subsequent  to  the  formation  of  the  fissure  and  probably  nearly  coinci- 
dent with  it  was  the  filling  of  the  wider  portions  with  intrusions  of  rhyolite, 
notwithstanding  the  fact  that  they  nowhere  quite  reach  the  surface  on 
Ruby  Hill. 

Evidence  goes  to  show  that  the  volcanic  energy  displayed  along  the 
fault  line  expended  the  greatest  activity  near  its  junction  with  the  great 
Hoosac  fault,  the  powerful  extravasations  of  rhyolite  gradually  dving  out 
toward  the  northwest,  and  beyond  the  intersection  with  the  Jackson  fault 
failed  to  overflow  the  top  of  the  fissure  walls.  The  rhyolites  exposed  in 


304  GEOLOGY  OP  THE  EUEEKA  DISTRICT. 

the  mines  rarely  attain  an  average  width  of  more  than  15  to  20  feet  across 
the  broadest  expansions,  although  instances  of  much  greater  width  occur  in 
the  Phoenix.  Decomposed  rhyolite  is  recognized  along  the  fissure  in  botli 
the  Jackson  and  Pho3iiix  mines.  It  is  intersected  by  the  Jackson  shaft 
above  the  third  level,  and  the  cross-cuts  from  the  old  Jackson  shaft  on  both 
the  third  and  fourth  levels  expose  the  rhyolite  body  oil  the  main  fissure. 
In  the  Phoenix,  rhyolite  is  found  on  all  the  lower  levels  wherever  they  inter- 
sect the  fissure.  Proceeding  westward  the  fissure  narrows,  but  the  rhyolite 
may  still  be  detected  on  the  sixth  level  of  the  KK,  although  so  thoroughly 
altered  as  to  have  lost  the  distinctive  characters  of  a  lava.  In  the  Rich- 
mond mine  no  rhyolites  nor  rhyolitic  clays  are  recognized,  nor  have  they 
been  observed  anywhere  along  the  fissure  to  the  northwest.  The  fissure 
gradually  narrows  and  finally  dies  out  and  the  fault  is  lost  where  the  rocks 
pass  beneath  Spring  Valley,  a  short  distance  beyond  the  Albion  mine. 

Transition  products  from  unaltered  lava  to  highly  kaolinized  rhyolite 
are  found  along  the  fissure  in  every  stage  of  decomposition.  In  the  Jack- 
son mine  the  rhyolite  origin  of  much  of  the  filling  of  the  fissure  is  deter- 
mined by  the  presence  of  mica  flakes  and  quartz  grains  imbedded  in  blue 
clay.  These  transition  products  grade  off  into  nearly  pure  clays  holding 
grains  of  quartz  still  unaltered,  whereas  the  feldspars  and  glass  base  have 
undergone  such  complete  kaolinizatiou  that  the  volcanic  origin  of  much  of 
this  material  could  not  be  made  out  but  for  its  association  with  fresher 
rock.  In  places  the  entire  filling  between  the  walls  of  the  fissure,  which 
may  be  only  a  few  inches  in  width,  is  composed  of  rhyolite  clays,  the 
extreme  product  of  the  action  of  steam  and  solfataric  fumes  upon  injected 
volcanic  rock.  They  have  all  the  physical  properties  of  and  behave  like 
ordinary  clays.  Between  the  Phouuix  and  the  Richmond  occur  bodies  of 
clay  which  are  undoubtedly  derived  from  the  rhyolite,  with  an  admixture 
of  more  or  less  calcareous  material.  Such  material  abounds  where  the 
fissure  walls  stand  only  a  few  inches  apart,  and  a  movement  has  pulverized 
the  limestone  along  the  fault  plane,  producing  an  admixture  of  rhyolitic 
clay  with  comminuted  siliceous  limestone.  In  the  Richmond  the  filling 
of  clay  between  the  fault  planes  is  derived  solely  from  attrition  of  the 
walls;  at  least,  no  rhyolite  can  be  detected.  It  is  possible  that  the  crack 


SECONDARY  FISSURE.  ;j(i;) 

became  too  narrow  to  permit  of  the  forcing  upward  of  the  liquid  lava 
without  sufficient  power  to  widen  the  space  between  the  inclosing  walls. 
Here  the  volcanic  quartz  grains  are  wanting,  the  calcareous  nature  of  the 
material  determining  its  origin. 

In  following  its  northwest  course  the  main  fissure  crosses  the  entire 
width  of  the  limestone  of  Ruby  Hill,  which,  by  means  of  the  network  of 
underground  workings,  may  be  easily  studied  and  compared  from  base  to 
summit  with  the  same  horizon  on  Prospect  Ridge.  Near  the  American 
shaft  the  distance  from  the  main  fissure  to  the  underlying  quartzite  is  only 
a  few  feet.  This  distance  increases  in  the  Phoenix  and  Jackson  mines  as 
proved  by  the  crosscuts  on  different  mining  levels,  the  limestone  belt 
gradually  becoming  wider  toward  the  west.  Near  the  Richmond  mine  the 
main  fissure,  having  traversed  the  limestone,  follows  the  contact  between 
Prospect  Mountain  limestone  and  Secret  Canyon  shale  for  a  considerable 
distance,  beyond  which  it  is  lost.  In  the  Jackson  mine  a  shale  belt  is 
exposed  which,  although  fairly  persistent  in  the  underground  workings  of 
the  KK,  Eureka,  and  Richmond  mines,  never  reaches  the  surface,  owing  to 
the  fault  across  the  limestone.  Without  much  doubt  the  shale  corresponds 
with  the  broad  irregular  shale  belt  found  on  Prospect  Ridge  and  designated 
the  Mountain  shale.  To  the  south  of  this  main  fissure,  along  the  contact  of 
the  Prospect  Mountain  quartzite  and  the  Prospect  Mountain  limestone, 
occurs  a  line  of  faulting  which,  although  of  less  magnitude  than  the  Ruby 
Hill  fault,  is,  on  account  of  its  relation  to  the  ore  bodies,  quite  as  important 
from  an  economic  point  of  view.  Like  the  Ruby  Hill  fault  it  was  formed 
subsequent  to  the  minor  displacements  connected  with  the  earlier  orographic 
movements.  Evidence  seems  to  show  that  this  faulting  took  place  contem- 
poraneously with  that  of  the  Ruby  Hill  fault  and  has  been  named  the  sec- 
ondary fissure.  This  secondary  fissure  possesses  an  average  dip  of  40°, 
coinciding  with  the  contact  plane  between  the  two  formations,  and  as  the 
ano'le  of  inclination  of  the  main  fissure  uniformlv  stands  at  70°,  the  two 

O  •» 

faults  might  naturally  be  expected  to  come  together  at  110  great  distance 
from  the  surface.  Exploitation  confirms  this  supposition  and  in  the  lower 
workings  of  the  mines  the  secondary  fissure  is  easily  traceable  into  the 
Ruby  Hill  fault,  but  has  nowhere  been  observed  to  cross  it.  Indeed,  this 

MON    XX 20 


30fi 


GEOLOGY  OF  THE  EUREKA  DISTRICT. 


secondary  fissure  may  be  considered  as  an  offshoot  from  the  more  per- 
sistent and  profound  Ruby  Hill  fault.  Where  the  quartzite  and  limestone 
show  a  tendency  to  curve  to  the  south  and  southwest,  following  around  the 
spur  of  the  mountain,  the  secondary  fissure  abandons  the  contact  plane  and 
with  a  northwest  course  enters  the  limestone,  leaving  a  block  of  the  latter 
rock  between  it  and  the  quartzite. 

HHv Phoenix  Mine 


Quartzite    Crushed  JJimes'tone    SfatLel      '.Rlyolitc 
.Litnestone 


Flo.  6 Cross-section  in  PboPnix  mine. 


Within  the  wedge-shaped  limestone  body  included  between  the  Ruby 
Hill  fissure  and  the  secondary  fissure  have. been  found  all  the  deposits  of 
ore  which  were  of  sufficient  value  to  repay  extraction.  Up  to  the  time  of 
the  present  investigation  all  exploitations  by  crosscuts  from  the  main  levels 


CAVES  AND  CREVICES.  3<)7 

outside  these  limits  have  failed  to  discover  any  accumulations  of  ore.  The 
limestone  on  the  north  side  of  the  Ruby  Hill  fault  presents  a  fairly  com- 
pact uniform  appearance  occasionally  well  stratified.  Between  the  two 
fissures  the  limestone  is  crushed  and  broken,  everywhere  showing  the  effect 
of  great  pressure  accompanied  by  movement.  Much  of  this  rock  indicates 
alteration  by  chemical  process  since  the  fracturing  and  displacement.  The 
limestone  south  of  the  secondary  fissure  is  for  the  most  part  black  in  color, 
siliceous  in  composition,  and  in  distinction  to  the  limestone  between  the 
fissures  uniform  in  structure.  It  is  more  easily  recognized  than  the  other 
belts  and  resembles  the  lower  strata  of  limestone  on  Prospect  Ridge.  By 
the  miners  the  limestone  beneath  the  secondary  fissure  is  known  as  the 
back  limestone ;  that  found  between  the  two  fissures  is  called  either  the 
crashed  or  mineral  limestone,  while  the  beds  overlying  the  main  fissure  are 
referred  to  usually  as  the  front  limestone. 

Figure  6  represents  the  relative  position  of  the  Ruby  Hill  fault,  along 
which  the  main  fissure  has  been  formed,  to  the  secondary  fissure  as  shown 
by  a  vertical  cross-section  in  the  Phoenix  mine.  It  will  be  seen  that  the 
two  fissures  come  together  just  below  the  sixth  level  of  the  mine.  The 
rhyolite  dike  follows  the  Ruby  Hill  fault,  and  nowhere  deviates  to  the 
southward  in  its  upward  course.  In  the  ground  shown  by  the  section  the 
secondary  fissure  adheres  closely  to  the  line  of  contact  between  the  quartzite 
and  limestone.  The  ore  body  is  cut  by  the  shaft  extending  from  the  surface 
nearly  down  to  the  point  of  contact  between  the  formations.  Near  the 
third  level  the  shaft  enters  the  underlying  quartzite  and  has  been  sunk  only 
a  short  distance  below  the  fifth  level,  the  sixth  being  reached  by  an  incline. 

Preexisting   Caves   and   Crevices.— It    has    1)6611    Stated    that    the    fissure    which 

accompanies  the  main  fault  on  Ruby  Hill  has  been  the  principal  channel 
through  which  the  intrusive  rhyolites  have  been  forced  upward  to  within  a 
short  distance  of  the  surface,  if,  indeed,  they  have  not  accumulated  on  top  and 
subsequently  been  removed  by  erosion.  On  the  other  hand,  the  secondary 
fissure  carries  no  rhyolite,  but,  accompanying  it,  especially  along  the  contact 
of  rhyolite  and  limestone,  are  large  and  valuable  bodies  of  ore.  Between 
these  two  fissures  the  crushed  limestone  shows  the  evidence  of  faulting 
approximately  parallel  with  the  Ruby  Hill  fault,  and  due  to  forces  acting  at 


308  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

the  time  the  main  ridge  was  uplifted.  Other  faults  indicate  lateral  thrust,  but 
they  are  less  effective  than  the  former  and  may  be  of  later  origin,  due  to  sub- 
terranean forces  connected  with  the  period  of  volcanic  energy.  Accom- 
panying these  are  innumerable  small  fissures,  seams,  crevices,  chambers 
and  channels  of  varied  shapes  and  sizes.  Many  of  these  owe  their  origin 
simply  to  the  dynamic  effects  of  upheaval.  Others  are  best  explained  on 
the  theory  of  surface  waters  percolating  downward  along  lines  of  least 
resistance,  widening  fissures  and  enlarging  cavities.  If  these  waters  were 
charged  with  carbonic  acid,  chambers  and  irregular  shaped  galleries  and 
drainage  channels  must  necessarily  have  been  dissolved  out  of  the  limestone. 
A  study  of  these  channels  and  their  intricate  connections  tends  to  the  belief 
in  the  theory  of  preexisting  caves  and  underground  water  courses  before 
the  introduction  of  ore. 

Fining  of  Fissures.— The  coming  in  of  the  volcanic  period  would  be  quite 
likely-  to  disturb  and  dislocate  any  previous  system  of  subterranean  drain- 
age, in  some  places  completely  closing  and  in  others  opening  new  channels 
by  the  formation  of  fresh  cracks  and  crevices.  Subsequent  to  the  penetra- 
ting of  the  main  fissure  by  rhyolite  came  the  filling  of  minor  fissures  and 
other  openings  in  the  limestone  by  the  ascending  mineral  solutions  and 
gaseous  currents.  Wherever  the  narrow  fissures  admitted  of  it  these  open- 
ings and  chambers  were  more  or  less  filled  with  mineral  matter  precipitated 
from  solution,  the  passage  ways  in  many  instances  being  left  neai'ly  ban-en 
or  only  carrying  stringers  and  slight  indications  of  earthy,  ocherous 
material  probably  deposited  before  the  dying  out  of  the  active  mineral 
currents.  In  some  instances  the  narrow  connecting  channels  between  the 
larger  openings  are  richer  in  mineral  matter  than  the  chambers  themselves. 
It  would  be  useless  to  speculate  on  the  reasons  why  certain  fissures  and 
chambers  earned  ores  and  others  were  left  barren.  The  freaks  of  deposi- 
tion from  ascending  currents — in  some  places  rapid,  in  others  slow — and  the 
varying  conditions  of  temperature  and  pressure  brought  about  by  the  vary- 
ing intensity  of  solfataric  action  would  produce  endless  differences  in  the 
mode  of  occurrence.  Channels  which  at  one  time  presented  conditions 
most  favorable  for  deposition  might  at  a  later  period  become  entirely  cut 
off  from  ascending  currents.  Anyone  who  has  observed  carefully  the 


AGE  OP  ORE  DEPOSITS. 

apparent  freaks  in  the  deposition  of  mineral  matter  in  such  centers  of 
thermal  activity  as  the  Yellowstone  Park  realizes  how  little  it  takes  to 
deflect  the  course  of  ascending  aqueous  or  gaseous  currents  and  how,  under 
varying  conditions,  mineral  deposition  is  liable  to  undergo  change  within 
restricted  areas.  The  occurrences  of  ore  bodies  on  Ruby  Hill  and  Pros- 
pect Mountain  are  variable  and  uncertain,  but  such  as  one  might  anticipate 
from  their  mode  of  formation. 

Relative  Age  of  Rhyolite  and  Ore.— The    best    example    Oil    Ruby   Hill    showing 

direct  contact  between  the  ores  and  rhyolite  bodies  was  observed  in  the 
Jackson  mine,  but  probably  a  still  finer  illustration  of  the  relationship 
between  the  two  with  the  ore  lying  undisturbed  along  the  under  side  of  a 
highly  inclined  dike  was  seen  in  the  Dunderburg  mine  on  Hamburg  Ridge. 
It  was  exposed  on  the  third  level  of  the  mine  near  the  main  shaft  which 
had  been  sunk  all  the  way  in  hard  limestone.  The  rhyolite  dike  varied 
from  1  to  8  feet  with  an  average  width  of  a  little  more  than  2  feet.  The 
strike  of  the  dike  was  approximately  east  and  west  with  a  dip  to  the  north, 
whereas  the  course  of  the  ore  channel  stood  nearly  at  right  angles  to  it. 
The  ore  never  penetrated  the  rhyolite,  its  course  being  deflected  on 
approaching  the  intrusive  dike.  At  the  shaft  house  the  ore  body 
measured  50  feet  in  thickness.  Opportunity  for  examining  the  contact 
was  excellent  as  much  of  the  ore  still  remained  in  place,  while  over 
other  areas  along  the  contact  the  ore  had  been  stripped  off,  rendering  it 
possible  to  observe  the  relationship  between  it  and  the  dike,  as  well  as  the 
position  of  both  to  the  inclosing  limestone.  Nowhere  did  the  ore  penetrate 
the  rhyolite,  and  nowhere  had  any  ore  been  found  inclosed  within  the  dike. 
In  like  manner  the  ore  was  wholly  free  from  rhyolite.  Nothing  could  seem 
more  clear  than  that  the  mineral  matter  had  been  quietly  deposited  from 
solution  along  the  under  side  of  a  highly  inclined  dike,  neither  could  any- 
thing be  seen  suggesting  a  replacement  of  rhyolite  by  ore,  although  imme- 
diately along  the  contact  there  is  considerable  kaolinization  of  rhyolite. 
Such  instances  as  the  Jackson  mine  on  Ruby  Hill,  the  Dunderberg  on 
Hamburg  Ridge,  and  the  position  of  the  rhyolite  and  ore  at  the  Geddes 
and  Bertrand  mine  in  Secret  Canyon,  furnish  strong  proof  corroborating 
other  evidence  that  the  ore  followed  the  rhyolite. 


310  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

Kaoiinization  of  Rhyoiite.— A  careful  study  of  the  transition  products  of 
kaolinization  of  the  rhyolite  shows  how  coinplete  the  decomposition  has 
been  along  the  Ruby  Hill  fissure  west  of  the  Jackson  fault.  Equally  com- 
plete and  impressive  are  the  evidences  of  similar  kaolinization  in  the  Dun- 
derburg  and  on  the  summit  of  Hamburg  Ridge  above  the  Dunderburg  and 
Hamburg  mines  wherever  the  rhyolites  offer  good  exposures  on  the  surface. 
Along  the  line  of  contact  on  the  summit  of  the  ridge  between  the 
thoroughly  whitened  rhyolite  and  the  dark  limestone  there  has  been  con- 
siderable prospecting  for  ores,  but  without  success.  Perhaps  the  best 
instance  of  the  alteration  of  the  rhyolite  is  found  where  the  drainage  chan- 
nel of  New  York  Canyon  in  coming  down  from  Prospect  Ridge  has  worn 
a  deep  passage  through  the  Hamburg  limestone  ridge.  It  is  seen  in  the 
limestone  bluff  on  the  south  side  where  an  exploring  tunnel  was  run  into 
the  hill  following  the  contact  between  the  nearly  vertical  rhyolite  dike  and 
the  inclosing  limestone.  There  is  exposed  here  a  fine  example  of  com- 
pletely kaolinized  rhyolite  possessing  all  the  properties  of  an  ordinary  clay, 
except  that  the  quartz  grains  of  rhyolite  still  remain  unacted  upon  with 
here  and  there  a  little  unaltered  sanidin.  This  is  an  instance  of  thoroughly 
kaolinized  rhyolite  without  the  presence,  so  far  as  known,  of  any  ore  body 
as  far  as  the  tunnel  was  run.  Finding  no  indication  of  ore,  the  tunnel  had 
been  abandoned  after  running  a  long  way  into  the  hill  along  the  contact 
of  the  two  formations. 

Ores  Deposited  as  Sulphides.— Solfataric  action  which  accompanied  the  filling 
of  the  intricate  net-work  of  openings  in  the  limestone  may  have  continued 
throughout  a  long  period  of  time,  the  mineral  matter  accumulating  slowly. 
That  the  ores  were  originally  deposited  as  sulphides  there  seems  no  good 
reason  to  doubt,  an  opinion  probably  held  by  all  geologists  who  have  exam- 
ined the  district  and  who  believe  that  the  ores  came  from  below. 

The  enormous  amount  of  oxidized  products  indicates  that  the  original 
ore  was  mainly  galena  and  pyrites.  Evidence  that  such  was  the  case  on 
Ruby  Hill  is  shown  by  the  discovery  of  fragments  of  galena  and  pyrites 
found  in  a  perfectly  fresh  state  scattered  throughout  the  ore  bodies  near 
the  surface  as  well  as  at  great  depths.  These  fragments  are  frequently  sur- 
rounded by  partially  oxidized  material  showing  a  nucleus  or  kernel  of  still 


NATUKE  OF  ORES.  311 

unaltered  sulphide.  Assuming  it  to  be  correct  that  the  ores  were  originally 
deposited  as  galena  and  pyrites,  it  is  most  difficult  to  see  how  such  vast  accu- 
mulations of  these  sulphides  could  have  been  formed  in  any  other  way  than 
in  the  preexisting  caves  and  openings.  Any  theory  with  which  we  are 
acquainted  of  chemical  and  physical  replacement  of  the  limestone  or  dolo- 
mite seems  wholly  inadequate  to  meet  the  necessary  conditions.  Pseudo- 
morphs  of  galena  and  pyrites  after  calcite  have  been  described  as  minera- 
logical  curiosities  and  possibilities,  but  nowhere  have  they  been  found  in  large 
quantities  in  any  mine,  and  so  far  as  the  writer  is  aware  they  have  never 
been  recognized  at  Eureka.  On  the  other  hand,  underground  drainage 
channels  probably  existed  before  the  deposition  of  the  ore  bodies,  and  with 
the  coming  in  of  the  ascending  mineral  currents  it  is  most  natural  that  they 
should  have  followed  these  channels  in  their  upward  course. 

The  Ores.— After  the  deposition  of  the  metallic  sulphides  came  the  period 
of  oxidation,  which  probably  continued  throughout  the  greater  part  of 
Quaternary  time  and  was  due  to  atmospheric  agencies,  mainly  percolating 
surface  waters.  On  Ruby  Hill  this  oxidation  may  be  said  to  be  nearly 
complete,  unaltered  galena  and  pyrites  being  exceptional  occurrences  above 
water  level.  It  has  produced  a  great  variety  of  secondary  minerals,  but 
such  as  it  might  be  anticipated  would  follow  the  complete  alteration  of  an 
admixture  of  argentiferous  galena  and  auriferous  pyrites  accompanied  by 
compounds  of  arsenic  and  molybdenum.  Mr.  Curtis  has  devoted  consider- 
able time  to  an  investigation  of  the  miueralogical  character  of  the  ore  and 
has  published  a  catalogue  of  the  minerals  known  to  occur  on  Ruby  Hill. 
The  secondary  products  of  oxidation  include  a  long  list  of  carbonates, 
sulphates,  arseniates,  molybdates,  and  chlorides.  The  ore  is  exceptionally 
rich  in  gold.  Wulfenite  occurs  in  brilliant  transparent  crystals,  varying  in 
color  from  lemon-yellow  to  bright  orange,  and  is  found  in  large  clusters 
filling  cavities  or  incrusting  other  minerals.  Since  the  opening  of  the 
mines  the  wulfenite  of  Eureka  has  been  much  sought  after  by  mineral- 
collectors  both  in  this  country  and  in  Europe.  It  appears  to  be  the  only 
species  which  Ruby  Hill  has  developed  that  has  any  exceptional  value  from 
a  mineralogical  point  of  view. 

The  following  analysis  of  a   sample  of  all  the  ores  smelted  at  the 


312  GEOLOGY  OF  THE  ECliEKA  DISTRICT. 

Richmond  furnace  tor  the  year   1878  was  made  for  the  company  by  Mr. 
Fred  Clandet,  of  London: 

Lead  oxide 35-65        Lead 33-12 

Bismuth 

Copper  oxide -15        Copper..  -12 

Iron  sesquioxide 34-39        Iron .24-07 

Zinc  oxide 2-37        Zinc 1-89 

Manganese  oxide -13 

Arsenic  acid 6-34        Arsenic 4-13 

Antimony -25        Antimony -25 

Sulphuric  acid 4-18        Sulphur 1-67 

Chlorine 

Silica 2-95 

Alumina -64 

Lime 1  -14 

Magnesia -41 

Water  and  carbonic  acid 10-90 

Silver  and  gold -10 

99-60 

Silver,  27'55  troy  ounces  per  ton  of  2,000  pounds.  Gold,  1'59  troy 
ounces  per  ton  of  2,000  pounds. 

The  analysis  is  taken  from  the  records  of  the  Richmond  company. 
It  is  reproduced  here,  as  it  gives  the  average  composition  of  a  large  quantity 
of  ore  probably  derived  from  the  same  ultimate  source,  and  is  therefore  not 
without  scientific  value,  although  it  cannot  be  considered  as  representing  any 
definite  deposit  or  the  product  of  any  special  mode  of  formation. 

The  method  of  stating  the  present  composition  of  the  ore  is  somewhat 
misleading.  All  the  lead  is  estimated  as  lead  oxide,  whereas  a  very  appre- 
ciable amount  of  lead  sulphide  must  have  been  present,  as  is  shown  by  the 
examination  of  any  ore  pile.  It  indicates,  however,  how  completely  the 
ore  body  has  undergone  oxidation  since  deposition.  No  determination  was 
made  of  the  molybdic  acid,  yet  it  is  hardly  possible  that  none  was  present 
when  it  is  easily  detected  in  almost  any  ore  sample.  The  low  percentage  of 
base  metals  other  than  lead  and  iron  shows  the  great  uniformity  and  sim- 
plicity of  the  original  sulphides.  That  the  ores  vary  in  composition  within 
certain  limits,  dependent  upon  the  position  of  the  ore  chamber  and  their 


LOUD    BYKON  AND    KELLY    MINES. 


313 


connection  with  the  fissures  and  pipes,  is  admitted  by  all  who  have  care- 
fully studied  the  deposits.  Occasionally  the  ore  contained  in  small  pockets 
in  the  limestone  will  present  a  fairly  uniform  composition,  but  differing 
widely  from  that  found  in  adjacent  bodies,  and  in  some  of  these  zinc  and 
copper  accumulate  in  relatively  large  quantities  as  compared  with  the 
entire  mass  of  ore.  Such  variations  appear  to  be  much  more  common  on 
Prospect  Ridge  than  on  Ruby  Hill. 

Two  of  the  most  singular  and  interesting  of  these  isolated  deposits 
were  found  near  the  summit  of  Prospect  Ridge,  on  two  adjoining  mining 
properties,  known  as  the  Lord  Byron  and  Kelly  mines.  They  resemble 
each  other  so  closely  that  they  may  very  properly  be  considered  as  having 
a  common  origin  and  possibly  filling  the  same  fissure,  the  connection 
between  them  being  concealed  beneath  the  surface. 

The  following  analyses  of  these  complex  ores  were  kindly  made  for 
the. writer  by  Dr.  W.  F.  Hillebrand,  of  the  U.  S.  Geological  Survey: 


Lord  Byron. 

Kelly. 

RisTmit.hnns  oxide     ...                                               ..... 

29-54 

40-023 

Lead  oxide  

1-97 

Tin  oxide  

0-273 

Telluric  acid  

12-69 

1-077 

A  n  t  iinoii  ions  oxide 

Not  determined. 

0  -752 

Copper  oxide                                                              ... 

1-00 

0-559 

Ferric  oxide  

15-30 

0-713 

1-350 

Alumina  

0-23 

0-259 

Zinc  oxide  

5-54 

0-350 

Uranium  oxide  

Not  determined. 

0-081 

Lime  

2-77 

20-650 

Magnesia                                                                     

Trace. 

5-691 

Carbonic  acid                                                             

2-56 

25-002 

Chlorine    . 

Not  determined. 

0-047 

Phosphoric  acid                                .              .  

0-24 

0-079 

Sulphuric  acid                                     ..       ..  

2-41 

0-520 

Silica    .                                              

15-31 

1-402 

Water          

3-39 

0-913 

Silver  

1-01 

0-034 

Gold  

0-001 

0-002 

Total  

93-961 

99-777 

314  GEOLOGY  OF  THE  EUltEKA  DISTRICT. 

Dr.  Hillebraud  regards  the  greater  part  of  the  tellurium  as  occurring 
in  the  state  of  telluric  acid,  because  in  boiling  with  hydrochloric  acid  much 
chlorine  is  evolved,  although  there  is  110  manganese  present,  and  on  reducing 
it  with  sulphurous  acid  the  tellurium  is  immediately  precipitated.  Dr.  Hille- 
brand  notes  the  absence  of  all  sublimation  products  of  tellurium,  antimony, 
and  arsenic  on  heating  the  ore  in  an  open  tube,  indicating  the  previously 
complete  oxidation  of  these  substances.  The  telluric  acid  is  nearly  sufficient 
in  the  Lord  Byron  ore  to  combine  with  the  bismuth,  leaving  enough  over 
to  combine  with  the  small  amount  of  silver  present.  Some  silver  in  both 
ores  probably  exists  in  combination  with  tellurium  as  a  telluride.  These 
ores  possessed  no  commercial  value  on  account  of  the  exceedingly  small 
amount  of  them  obtained.  In  the  case  of  the  ore  from  the  Kelly  mine 
it  will  be  seen  that  there  is  a  large  amount  of  carbonate  of  lime  present, 
but  it  is  not  possible  to  say  whether  this  occurred  as  calcite,  a  secondary 
product,  or  as  limestone  derived  from  the  country  rock. 

Occasional  pockets  or  crevices  in  the  limestone  are  characterized  by 
the  deposition  of  wad  and  other  compounds  of  manganese.  Silica  is  rarely 
found  in  the  fissures  and  chambers  in  the  Richmond  and  Eureka  mines,  but, 
on  the  other  hand,  most  of  the  deposits  on  Prospect  Ridge  carry  more  or 
less  quartz,  and  associated  with  it  there  appears  to  be,  judging  from  the 
assay  reports,  an  increase  in  the  amount  of  gold.  Silica  also  characterized 
the  mines  of  the  Hamburg  Ridge,  as  is  shown  not  only  in  some  of  the 
larger  deposits,  but  in  such  properties  as  the  Connolly  and  California  mines. 
Variations  in  silica  present  no  greater  range  than  other  mineral  matter 
deposited  under  conditions  of  solfataric  action.  It  is  known  as  one  of  the 
most  common  products  from  deep-seated  sources.  The  noticeable  feature 
about  the  silica  is  not  its  occurrence  on  Prospect  Ridge,  but  rather  its 
absence  from  the  Ruby  Hill  fissure  and  connecting  chambers. 

As  has  been  previously  mentioned,  the  mines  of  Ruby  Hill  have 
yielded  up  to  the  time  of  the  present  investigation  gold  and  silver  to  the 
value  of  $60,000,000,  accompanied  by  over  a  quarter  of  a  million  tons  <>t 
commercial  lead.  The  large  amount  of  iron  contained  in  the  ores  has 
never  been  estimated.  These  enormous  products  of  the  heavy  metals, 
deposited  in  small  openings  in  the  Ruby  Hill  limestone,  within  a  very 


RECENT  CHANGES.  315 

limited  area,  through  narrow  fissures,  from  some  deep-seated  source,  cer- 
tainly present,  in  their  scientific  aspects,  most  interesting  problems  to  the 
geologist. 

Recent  changes.— Following  the  oxidation  of  the  sulphides  and  in  some 
degree  associated  with  it,  came  the  partial  rearrangement  of  the  oxidized 
material  under  the  influence  of  percolating  surface  water,  in  some  instances 
removing  the  ore  from  narrow  passageways  and  fissures  and  sweeping  it 
into  larger  receptacles,  where,  together  with  ore  already  deposited,  it  was 
piled  up  on  the  limestone  floors  This  rearrangement  of  the  oxidized 
material,  seen  upon  opening  several  chambers  on  Ruby  Hill  in  the  course 
of  mining  exploration,  appeared  so  self-evident  that  no  other  theory  of  their 
formation  seems  adequate  to  account  for  all  the  observed  facts.  Such  ore 
receptacles,  although  more  frequent  near  the  surface,  were  opened  at  differ- 
ent depths,  but  usually  along  lines  that  gave  every  appearance  of  having 
been  ancient  water  courses.  On  opening  a  chamber  the  tops  of  the  ore 
piles  would  be  found  covered  by  an  accumulation  of  dust,  fine  sand,  and 
material  foreign  to  the  ore  body.  The  stratification  of  material  shown  in 
cross  section  and  the  settling  of  the  heavier  particles  under  the  action  of 
water  were  too  convincing  to  admit  of  any  other  mode  of  formation.  Many 
of  the  pipes  coming  down  from  the  surface  would  be  found  wholly  barren 
of  ore,  yet  carrying  fragments  of  limestone  rounded  and  worn  smooth  by 
he  action  of  percolating  subterranean  waters  charged  with  carbonic  acid. 
Such  recent  drainage  channels  and  pipes  are  by  no  means  restricted  to 
Ruby  Hill,  but  occur  equally  well  preserved  on  Prospect  Mountain,  and 
may  be  seen  with  more  or  less  distinctness  in  some  of  the  tunnels  cutting 
the  ridge.  They  are  well  shown  in  both  the  Eureka  and  Prospect  Moun- 
tain tunnels  and  are  so  large  as  to  serve  the  purposes  of  ventilation,  and 
in  one  instance,  at  least,  so  straight  as  to  admit  light  from  the  surface. 
Many  of  the  tunnels  suggest  the  existence  of  subterranean  water  courses 
at  a  time  when  the  country  was  less  arid  than  at  present 

Dr.  J.  S.  Newberry1  has  cleverly  suggested  that  climatic  changes,  with 
alternating  wet  and  dry  periods,  within  Quatemary  time  in  the  Great  Basin, 
may  have  had  much  to  do  in  determining  water  levels  in  deep  mines.  If 

1  School  of  Mines  Quarterly,  March.  1880. 


316  GEOLOGY  OF  T11E  EtJltEKA  D1STEIOT. 

this  is  true,  the  action  of  percolating  waters  in  underground  drainage  chan- 
nels would  be  influenced  in  like  manner.  During  the  present  period  of 
excessive  dryness  these  channels  in  the  limestone  carry  no  water,  and  con- 
sequently exert  but  little  solvent  power.  If,  however,  subterranean  cham- 
bers can  be  worn  in  the  limestone  since  the  deposition  of  the  ore;  it  seems 
but  logical  to  assume  that  on  the  identical  ground,  under  nearly  similar 
conditions,  caves  should  have  been  formed  before  the  deposition  of 
sulphides. 

Since  the  formation  of  these  more  recent  water  courses  nothing  of  any 
moment  occured  011  Ruby  Hill  until  historical  time,  when  man,  in  his  eager 
search  for  wealth,  excavated  in  a  few  years,  by  means  of  modern  mechan- 
ical appliances,  the  enormous  mineral  product  which  required  untold  ages 
to  deposit  by  natural  process. 

Conclusions.— The  conclusions  reached  after  an  investigation  of  the 
Eureka  Mountains  with  regard  to  the  geological  position,  age,  and  origin  of 
the  ore  deposits  may  be  briefly  stated  as  follows: 

The  rocks  in  which  the  ores  occur  are  sedimentary  beds  belonging  to 
the  Cambrian,  Silurian,  and  Devonian  periods. 

The  ores  were  deposited  after  the  eruption  of  the  rhyolite,  and  conse- 
quently they  are  of  Pliocene  or  post-Pliocene  age. 

In  their  mode  of  occurrence  the  ores  are  closely  associated  with  the 
dikes  of  rhyolite,  although  there  is  no  evidence  to  show  that  they  were 
derived  from  them. 

The  ores  came  from  below. 

They  were  for  the  most  part  deposited  as  sulphides  in  preexisting  caves 
and  cavities. 

They  were  oxidized  by  atmospheric  agencies,  mainly  surface  waters 
percolating  through  the  rocks. 


APPENDIX 


SYSTEMATIC  LIST  OF  FOSSILS 


OF 


EACH  GEOLOGICAL  FORMATION  IN  THE  EUREKA  DISTRICT,  NEVADA. 


BY 

CHARLES   DOOLITTLE   WALCOTT. 


317 


APPENDIX   A. 

SYSTEMATIC  LIST  OF  FOSSILS  FOUND  AT  EUREKA,  NEVADA. 


BY  CHARLES  D.  WALCOTT. 


In  the  monograph  of  Mr.  Arnold  Hague  the  grouping  of  the  different  genera 
and  species  and  their  stratigraphical  succession  and  relations  throughout  the  great 
thickness  of  sediments  at  Eureka  are  given  with  considerable  detail  in  the  discussion 
of  the  Paleozoic,  rocks.  For  the  student  of  general  geology  the  vertical  range  of 
species  and  their  geographical  distribution  are  (dearly  brought  out. 

For  the  purpose  of  bringing  together  in  tabulated  form  all  the  genera  and 
species  of  each  important  group  into  which  the  rocks  have  been  divided,  the  following 
systematic  list  was  originally  published  in  the  Paleontology  of  the  Eureka  District,1 
and  is  reproduced  here  in  order  that  the  student  may  see  at  a  glance  the  life  of  each 
geological  horizon. 

Invertebrate  life  is  well  represented  throughout  the  entire  series  of  rocks  from 
the  base  of  the  Prospect  Mountain  limestone  to  the  summit  of  the  Upper  Coal- 
measure  limestone,  a  thickness  of  over  28,500  feet  of  sediments. 

In  the  list  all  the  Cambrian  fauna  is  included  under  the  head  of  Prospect 
Mountain  group,  embracing  the  Prospect  Mountain  limestone,  Secret  Canyon  shale, 
Hamburg  limestone,  and  Hamburg  shale. 

Since  the  publication  of  the  Paleontology  the  Cambrian  fauna  has  been  divided 
into  three  subfaunas:  Lower,  Middle,  and  Upper.  Under  this  classification  the 
Lower  Cambrian  or  OleneUiot  fauna  is  included  in  the  quartzites  and  immediately 
superjaceut  shales  beneath  the  Prospect  Mountain  limestone;  the  Middle  Cambrian 
fauna  in  the  Prospect  Mountain  limestone  and  Secret  Canyon  shales,  and  the  Upper 
Cambrian  fauna  in  the  Hamburg  limestone  and  Hamburg  shales.  As  the  monograph 
of  Mr.  Hague  does  not  deal  with  these  fauna!  subdivisions  in  detail  no  further  refer- 
ence will  be  ma.de  to  them. 

A  number  of  generic  references  will  be  chauged  in  a  forthcoming  review  of  the 
Middle  and  Upper  Cambrian  faunas,  but  it  is  not  thought  best  to  anticipate  these 
changes  in  the  present  systematic  list. 

1  Paleontology  of  the  Eureka  District,  Nevada.     Hon.  XT.  S.  Gool.  Surv.,  vol.  vm,  18M. 

318 


320 


GEOLOGY  OF  THE  EUREKA  DISTRICT. 


The  Lower  Silurian  embraces  the  fauna  of  the  Pogonip  limestone  beneath  the 
Eureka  quartzite,  and  the  Silurian  fauna  above  the,  quartzite  is  represented  by  the 
meager  collections  obtained  from  the  Lone  Mountain  limestone. 

Under  the  grouping  of  Carboniferous  fossils  are  included  the  fauna  from  the 
great  belts  of  limestone  both  above  and  below  the  Weber  conglomerate. 

Included  in  the  list  is  the  collection  obtained  from  the  White  Pine  Mountains, 
situated  about  40  miles  southeast  of  Eureka.  The  fauna  from  the  base  of  the 
Pogonip  to  the  summit  of  the  Devonian  is  so  closely  related  to  that  of  the  Eureka 
District  that  they  may  well  be  studied  together,  and  in  the  tables  are  arranged  in 
parallel  columns. 

The  reader  who  desires  more  detailed  information  in  regard  to  the  specific 
character  of  the  fauna  is  referred  to  the  descriptions  given  in  the  Paleontology  of  the 
Eureka  District,  above  mentioned. 


SYSTEMATIC  LIST  OF  FOSSILS  OF  EACH  GEOLOGICAL  HORIZON. 

CAMBRIAN. 

PROSPECT  MOUNTAIN  TEBRANE. 

[The  clonlile  multiple  (XX)  denot«a  that  the  species  passes  to  the  group  above.] 


Genera  and  species. 

Eureka. 

White 
Pine. 

Remarks. 

Porifera. 

X 

• 

Type  from  the  Cambrian  of  Wales. 

Fragment. 
Types  from  Eureka  and  White  Pine  dis- 
tricts, Nevada. 
Type  from  Eureka  District. 
Type  from  Schell  Creek  Range,  Nevada. 
Type  from  Eureka  District. 
Like  O.  pretiosa  Billings. 

Type  from  Calciferous  formation  in  New- 
foundland. 

Type  from  Gallatin  River,  Montana. 
Type  from  Eureka  District. 

Type  from  St.  Croix  (Potsdam)  sandstone 
of  Wisconsin. 

sp.  ?..                         

X 

Brachiopoda. 

X 
XX 

XX 
XX 
XX 

X 
X 
XX 

X 
X 
X 
XX 

X 
X 

X 
X 

X 

?  minuta  H.  &  W  

X 

Obolella  discoidea  H.  &  W 

Acrothele  f  dichotoma  Walcott  

Kutorgina  prospectensis  Walcott  

sculptilis  Meek  ..  

whitfieldi  Walcott  .. 

Orthis  eurekensis  Walcott 

X 

Pteropoda. 

Hyolithe8  primordialis  Hall  (sp.)  ...  . 

Scenella?  conula  Walcott.  . 

SYSTEMATIC  LIST  OF  FOSSILS. 


321 


CAMBRIAN"— Continued. 
PROSPECT   MOUNTAIN   TKRRANK — Continued. 


Genera  and  species. 

Eureka. 

White 
Pine. 

Remarks. 

Pcecilopodu. 

X  X 

x 

X  X 

x 

Type  from  White  Pine  District   NevaU'i 

neon  H.  &  W 

XX 

x 

x 

x 

x 

howelli  Meek         

x 

Type  from  Pioche,  Nevada 

iddingni  Walcott  

x 

Olenoides?  expausus  Waleott  .  .  . 

x 

DicellocephaluH?  angustifrons   Walcott 

x 

?  bilobatus  H.  &  W    . 

x 

Type  from  Eureka  District 

flabellifer  H.  &  W 

T 

Geol.  Expl.  Fortieth  Par      vol   IV   p   227 

iole  Walcott..       .     . 

x 

inurica  Walcott 

x 

Ptychoporia  (f)  angulatus  H.  &  W 

x 

(ieol.  Expl.  Fortieth  Par.,  vol    iv  p  220 

frichmondensis  Walcott 

x 

anytus  H.  &  W 

x 

Type  from  Schell  Creek,  Nevada 

(S.)  breviceps  Walcott  

x 

?  grauulosns  H.  &.  W  

XX 

Type  from  Eureka  District. 

hiK'uei  II.  &  W.  (sp.) 

XX 

Type  from  White  Pine  District   Nevada 

la'.viceps  Walcott 

x 

(  ?)liunarssoni  Walcott  

x 

?niaciilosus  H.  &  W  (sp.).. 

XX 

Geol.  Expl.  Fortieth  Par.,  vol.  iv,  p.  215. 

nitidus  H.  &  W  (sp.)  

x 

Type  from  Eureka  District. 

occidcntalis  Walcott  

x 

oweui  M.  &  H  (sp.  )  .. 

X  X 

Tvpe  from  Big  Horn  Mountains,  Montana. 

S?)  prospectensis  Walcott  .. 

x 

?)  similis  Walcott  

x 

(  ?)  similis,   var.   robustus, 

x 

Walcott. 
(?)  unisulcatus  H.  &  W.  .. 

XX 

Type  from  Eureka  District. 

(Euloma?)  affinis   Walcott. 

XX 

(Euloma?)  dissimilis  Wal- 

x 

cott. 
(P.)  laticeps  H.  &  W  

X  X 

Type  from  White  Pine  District,  Nevada. 

(P.)  occidens  Walcott  ..... 

x 

8p.  ?  

x 

x 

?  pernasutus  Walcott  .  

uasutus  Walcott  

x 

osceola  Hall  ..  

x 

Type  from  St.  Croix  (Potsdam)  sandstone 

f  quadriceps  H.  &  W  ....  

x 

of  Wisconsin. 
Tvpe  from  Ute  Peak,  Was.iteh  Range,  Utah. 

x 

Tvpe  from  St.  Croix  (Potedani)  sandstone 

pustiilosa  H.  &  W  

L. 

of  Wisconsin. 
Geol.Expl.  Fortieth  Par.,  vol.  iv,  p.223. 

Chariocephalus  ?  tumifrons  H.  &  W  

X 

x 

L. 

Type  from  White  Pine  district,  Nevada. 

X  X 

x 

x 

IlUenurus  sp.?    ..  . 

X 

NOTE.— Sjiccies  from  White  Pino  District  occur  »t  the  base  of  tlie  Pogonip  group  and  are  doubtfully  referred  I"  the  Cambrian. 
MON  XX 21 


322  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

LOWER   SILURIAN  (Ordovician). 

POGONIP   TERRANE. 

[The  letter  C  in  the  first  column  denotes  that  the  species  also  occnrs  in  the  Cambrian.] 


Genera  and  species. 

Eureka  Lower. 

Eureka  Upper. 

1! 

^* 

<S   V 

«  b  P. 

sss 

i5 

Remarks. 

Rhizopoda. 
Receptaculites  ellipticus  Walcott 

X 

elongatus  Walcott 

X 

X 

u 

Hydrozoa. 
Graptolitlms  sp.  ?  .  .  .  

X 

Like  G.  bifidus  HalL 

Actinozoa. 
Monticulipora  sp.  ?        ..   

x 

/ 

I'olyzoa. 
Ptilodict  va,  sp.  ?  

x 

Brachiopoda. 
Lingulepis  mjpra  H.  &  W 

c 

fminuta  H.  &  W 

C 

districts. 

Lintnila  ?  uiauticula  White 

c 

sp.  ?  

L 

Obolella  ?  ambigua  Walcott  

x 

discoidea  H.  &  W  . 

c 

L 

Type  from  Eureka  District 

Acrotreta  gemma*  Billings  .... 

c 

L 

sp.  ?  

x 

fbundland. 
Like  A.  subconica  Kutorga. 

Schizambon  tvpicalis  Walcott  

x 

Lfpta'na  melita  H.  &  W 

c 

Strophomeua  nemea  H.  &  W  

x 

L 

Type  from  White  Pine  District  Nevada 

Orthis  haniburgensis  Walcott  

x 

lonensis  Walcott  

x 

perveta  Conrad  . 

x 

u 

Trenton  of  Wisconsin  Chazy  of  Canada 

pogonipensis  H.  &,  W.   .. 

x 

u 

Geol  Expl  Fortieth  Par    vol  iv  p  23 

testudinaria  1  )alni;ni 

x 

u 

tricenaria  Conrad  

x 

u. 

Canada,  etc. 
Trenton   group    species  of  New   York 

sp.  ?  . 

x 

Canada,  etc. 



Sp.  !     

x 

u. 

Streptorhynchus  minor  Walcott  

x 

I'orambonites  obscurus  H.  &  W  

x 

Geol  Expl.  Fortieth  Par    vol  IV  p  234. 

Triplesia  calcifera  Billings  

x 

Lamellibrancli  in  In  . 
Telliuoinya  contracta  Salter  ?  .    .  . 

x 

u 

?  hamburgensis  Walcott 

x 

Modiolopsis  occidens  Walcott.  . 

x 

u 

pogonipensis  Walcott  

x 

u. 

sp.  ?  .  . 

tr. 

*  Identified  by  Mr.  Meek,  from  Malade  City.    Hayden's  Report  for  1872,  p.  464. 


SYSTEMATIC  LIST  OF  FOSSILS. 


LOWER  SILURIAN  (Ordovidan)— Continued. 
POGONIP  TERRANE — Continued. 


Genera  and  species. 

Eureka  Lower. 

Eureka  Upper. 

si 

«  * 

-  •-  -- 

-    y, 

sis 

^ 

Remarks. 

Gasteropoda. 

u. 

Trenton  group  species    in  New  York, 
Canada,  etc. 
Like  B.  allegoricus  White. 

Trenton  group  of  Wisconsin. 

Trenton  group  species    in  New  York, 
Canada,  etc. 

Typo  from  the  White  Pine  District,  Ne- 
vada. 

A  sp.  of  the  Lower  Trenton  in  New  York. 

Species  of  Lower  Trenton  in  New  York 
State. 
Trenton  group  sp. 

Type  from  the  Schell  Creek  Range,  Ne- 
vada. 

Type  from  Gallatin  River,  Montana. 
Type  from  White  Pino  District,  Nevada. 

Gcol.  Expl.  Fortieth  Par.,  vol.  iv.,  p.  231. 
Geol.  Espl.  Fortieth  Par.,  vol.  iv.,  p.  226. 

sp.  ? 

X 

Straparollus  sp,  ?  

u. 

Rapkistoma  uasoni  Hall  (sp.)  

X 
X 

sp.  ?  

MurctuHonia  milleri  Hall  

u. 

HP.  ? 

X 

v  :  
Sp.  T  .    .    .    . 

u. 
u. 

PI  euro  toni  aria  lonensis  Walcott  .   

X 
X 
X 
X 
X 
X 
X 

sp.  ?  

Helicotoma  sp.  ?  

Miiclurea  annulata  Walcott 

u. 

carinata  Walcott 

subannulata  Walcott 

sp  ? 

Metoptoma  ?  analogii  Walcott  

u. 
u. 

phillipsi  Walcott  .  .         . 

C  vrtolites  siuuatus  H.  &  W  

X 

X 
X 

I'teropoda. 
t'oleoprion  minuta  Walcott  

u. 

Hyolithes  vanuxemi  Walcott 

sp.?.., 

u. 

u. 
u. 

Cephalopoda. 
Orthoceras  multicameratum  Hall  

X 

4  sp.?  

Endoceras  (like  E.  multitubulatum)   

X 

X 

X 

X 
X 
X 

proteifonne  Hall  

Crustacea. 
Leperditia  bivia  White  

sp.  ?  

Hey  richia  sp.  ?  

Plumulites  sp  ? 

Poecilopoda. 
A^nostus  bidons  Meek  

C. 

coiiimuiii.s  H.  &  W  .  

C. 

X 
X 

Dicellocephalus  inexpectans  Walcott  
tiualis   Walcott 

X 

multiciuctus  H.  &  W...        x 

324 


GEOLOGY  OF  THE  EUEEKA  DISTRICT. 

LOWER    SILURIAN  (Ordovician)— Continued. 

POGONIP  TERBANE — Continued. 


Genera  and  species. 

Eureka  Lower. 

Eurekal'pper. 

"•d 
*** 

^S£ 

-  •:  £~ 

-e  o  S 

rp 

Remarks. 

l'u  til  opoda  —  Continued. 

X 

granulosus  H.  &  W.  (sp.  ) 

C. 

Type  from  Eureka  District. 

huguei  H.  &.  W.  (sp.) 

C. 

Type  from  White  Pine  District. 

maculosus  H.  &  W.  sp 

C. 

Geol.  Expl.  Fortieth  Par.,  vol.  iv.,  p.  215. 

oweui  Meek  ... 

C. 

Type  from  Big  Horn  Mountains,  Mon- 

uniKiilcatus H.  &  \V  (sp.) 

c. 

tana. 

(Eulomal)  affinis  Walcott.. 
Art'tlnisinu  aniericana  Walcott 

0. 



L. 

Bathyurus  ?  congeiieris  Walcott  

x 

rogonipeiisis,  H.  &  W  .... 

U. 

Geol.  Expl.  Fortieth  Par.,  vol.  iv.,  p.  243. 

si  in  i  ]]  ini  us   Walcott  

x 

?  tuberculatns  \Valcott 

x 

U. 

fsp.f  

U. 

Cyphaspis  ?  breviiuarginatus  Walcott 

x 

Amphion  uevadensis  Walcott     .   . 

x 

U. 

so.  f   . 

x 

sn.f 

u 

C'eraurus  sp.  ?  

x 

Symphysurus  ?  goldfussi  Walcott 

x 

Barraudia  ?  maccoyi  Walcott 

x 

?  sp 

x 

x 

L. 

U. 

sp  f 

x 

Asaphus  caribouensis  Walcott  .  

x 

curiosus  Billings     ..    

x 

Type  from  Quebec  group  of  Canada. 

3  gp.  1             .  .              

x 

LONE   MOUNTAIN   SILURIAN. 


Genera  and  species. 

GO  o- 

j°'C 
o 

S3  d 

r 

White  Pine. 

Remarks. 

Actinozoa. 
Stroptelasnia  Walcott  ...        ..  .  .. 

x 

sp.f  

x 

/aphrentisf  sp.f  

x 

Hal  vsites  catenulatus  Linn  (sp.  )  

x 

u 

Mouticulopora,  sp  f  .  .  .. 

x 

Echinodermala. 
Cvstid.. 

X 

Separate  plates. 

SYSTEMATIC  LIST  OF  FOSSILS. 
LONE  MOUNTAIN  SILURIAN— Continued. 


325 


Genera  and  species. 

^3 

33  • 

|l 
o 

a 
S  ^ 
fe  3 

a 

S 

Remarks. 

Brachiopoda. 
Leptsena  sericea  Sowerby  

X 

Orthis  

T  .,     „      ,.     .  „ 

Cephalopoda. 
Orthoceras  sp.?  

Cyrtoceras  sp.  f  

,. 

Pcecilopoda. 
Ceraurus  

Dalmanites  

j. 

Trinuclens  concentricus  Murch  .  .  . 

». 

llla'iius  

Asaphns  platycephalus  Stokes  

DEVONIAN. 


[Abbreviations. — Up.  Held.  —  Upper  Helderborg.     Ham.  =  Hamilton  group.    Ch.  =  Chemung  group  of  the  Devonian 
eries  of  the  Geological  Survey  of  New  York.     fl.  &  W.— Hall  &  Whitneld,  Geol.  Expl.  Fortieth  Parallel,  vol.  IV,  1877.] 


Genera  and  species. 


.Port/era. 

Palicomanon  roemeri  Walcott 

Astylospongia  sp  ? 

•Stromatopora,  sp  ? 


Actinozoa. 


Fistulipora,  sp? 

Favosites  hemispherica  Y.  &  S 

basaltica,  var 


I 

o 

HJ 


X 

X 
X 


X 
X 


X 
X 


& 
B 
p 


.-s  a 


Alveolitos  multilamellataMeek . 

rockfordensis  Hall  f 

Cladopora  prolifica  Hall 

pulchra  Rominger  i 

sp.? X 

Thecia  raniosa  Rominger? x 

Syringopora  hisingeri  Billings 

perelegans  Billings X 

Aulopora  serpens  Goldfnss ;     X 

Cyathophyllum  corniculnm  M.  1 


,  Ed. 


rugosumM.  Ed.  A  Haiue. 


x 

X 

x 

x 

x 

x 

x 

Remarks. 


Up.  Held,  and  Ham.  of  New  York  and 
Canada;  Up.  Held.,  Falls  of  Ohio. 

Geol.  Expl.  Fortieth  Par.,  vol.  iv,p.  25. 
Type  from  Devonian  of  Iowa. 
Up.  H<-ld.  of  New  York  and  Falls  of  Ohio. 
Up.  Held,  of  New  York  and  Kails  of  Ohio. 
Up.  Held,  of  New  York  and  Falls  of  Ohio. 
Up.  Held,  of  New  York  and  Falls  of  Ohio. 
Up.  Held,  of  New  York  and  Falls  of  Ohio. 

Hani,  of  Michigan. 

Up.  Held,  of  New  York.  Canada.  Fall* 

of  Ohio,  etc. 
Up.  Held,  of  New  York,  Falls  of  Ohio, 

etc. 


826 


GEOLOGY  OP  THE  EUEEKA  DISTRICT. 

DEVONIAN— Continued. 


Genera  and  species. 


Actinosoa — Continued. 

Cyathophyllum  davidsoni  M.  Ed  . 

2n.  sp 

sp.? 

Acervularia  pentagona  Goldfuss. . 


Smithia  hennahii  Lousdale  (sp) ' 

Pachyphyllnm  woodman!  White,  sp j 

Diphyphylluui  simeoense  M.  Ed I     X 

I'tychophyllum  ?  infuudibuluni  Meek I 

Cystiphyllum  ainericanum  M.  Ed x 


2  n.  sp 

Polyzoa. 


Fenestella,  2  sp.  ? 
Thamniscns  ?  sp  ? 


Brack  iopoda. 

Lingtila  alba-pineusis  Walcott 

liena  Hall 

ligea  Hall 

ligea,  var.  nevadensis  Walcott 

lonensib  Walcott 

melie  Hall 

whitei  Walcott 

sp.  f 

Discina  lodensis  Hall 

minuta  Hall?.. 


sp. 


Pholidops  bellula  Walcott 

quudrangularis  Walcott 

Orthis  impressa  Hall 

nmcfailanei  Meek 


X 


X 

X 


X 
X 
X 


tulliensis  vanuxem  

Skenidinm  devonicum  Walcott 

Streptorhynchua  cbemungensis  Conrad 

(sp.) 
chemungensiSjVar.  pan- 

dora  Billings, 
cheinungensis,  var.  per- 

versa  Hall. 

Strophomena    rhomboidalis    Wilckens, 
(sp.) 

Strophodonta  arouata  Hall 

calvini  Miller 

canace  H.  &  W 


deraissa  Conrad  (sp.) 

ineqniradiata  Hall 

patersoni  Hall 

perplnna  Conrad  (sp.) ^ 

punctulifera  Conrad  (sp.).i     X 


x 
X 
X 

X 
X 


X 
X 

X 
X 
X 

X 
X 
X 


X 
X 
X 


X 
X 


X 

X 


X 
X 


X 
X 


X 
X 


X 

X 


X 


Remarks. 


Ham.  of  Iowa. 


Identified    by   Meek  from  White   Pine 

District,  Nevada. 

Geol.  Expl.  of  Fortieth  Par.,  vol.  iv.,  p.  32. 
Devonian  of  Iowa. 

Up.  Held,  of  New  York,  Falls  of  Ohio,  etc. 
Geol.  Expl.  Fortieth  Par.,  vol.  iv,  p.  28. 
Up.  Held,   of  New  York  and  Falls  of 

Ohio ;  Ham.  of  New  York,  etc. 


Ham.  of  New  York. 
Ham.  of  New  York. 


Pal.  of  New  York,  vol.  iv,  p.  14. 


Ham.  of  New  York. 
Ham.  of  New  York. 


Ch.  of  New  York. 

Type    from    Mackenzie  River,    British 

America. 
Ham.  of  New  York. 

Up.  Held.,  Ham.,  and  Ch.  of  New  York. 
Up.  Held,  of  New  York  and  Canada. 
Up.  Held,  and  Ham.  of  New  York. 
Up.  Held.  New  York,  Falls  of  Ohio,  etc. 

Devonian  of  Iowa. 

Devonian  of  Iowa. 

Geol.  Expl.  Fortieth  Par.,  vol.  iv,  p.  246, 
1877. 

Devonian  of  Iowa,  New  York,  etc. ;  Mac- 
kenzie River,  British  America. 

Up.  Held,  of  New  York. 

Up.  Held,  of  New  York. 

Throughout  the  Devonian  of  New  York. 

Throughout  the  Devonian  of  New  York. 


SYSTEMATIC  LIST  OF  FOSSILS. 
DEVONIAN— Continued. 


327 


Genera  and  species. 


Brachiopoda — -Continued. 

Cnonetes  deflecta  Hall 

filistriata,  Walcott 

hemispherica  Hall 

macrostriata  Walcott . . 
mucronata  Hall 


setigera  Hall. 
HP. 


X 
X 

X 

X 


X 


Productus  (P. )  hallanus  Walcott 

(P.)  hirsutiforme  Walcott 

(P.)  lachrymosns  Conrad  (up.) 
(P.)  laehrymosus  var.   limus 

Conrad  (sp.) 
(P.)  lachrymosus,    var.    stig- 

matus  Hall. 

(P.)  navicellus  Hall X 

(P.)  shuuiardianus  Hall X 

(P.)  shumardianus  var.  pyxi- 

datus  Hall. 

(P. )  speciosus  Hall 

(P.)  subaculeatus  Murch 

(P. )  truncatus  Hall 

sp.? 

Spirifera  alba-pinensis  H.  &  W 

disjuucta  Sowerby 


englemanni  Meek  ? . 


parryana  Hall  f 

pifionensis  Meek 

raricosta  Conrad  (sp.) 

strigosus  Meek 

varicosa  Hall 


sp.  uudt 

(M.)   glabra,   nevadensis  Wal- 
cott. 

(M.)  maia  Billings 

(M. )  undifera  Roemer 

Spiriferina  cristata  Schlotheim  (sp. ) 

Amboccelia  umbonata  Conrad  (sp. ) 

Cyrtina  davidsoni  Walcott 

hamiltouensis  Hall 

Nucleospira  concinna  Hall 

Trematospira  infrequens  Walcott 

Retzia  radialis  Phillips  (sp.) 

Athyris  angelica  Hall 

sp.? 


Meristella  (W.)  nasuta  Conrad  (sp.) 

At  ry pa  desquamata  Sowerby 

reticularis  Linn,  (sp.) 
Uhynchonella  castanea  Meek 


duplicata  Hall 

emmonsi  H.  &  W. 
horstbrdi  Hall  .. 


X 

X 
X 


X 

X 


> 

X 


X 
X 


X 
X 
X 
X 

\ 


X 
X 


X 
X 


X 
X 


X 
X 


X 

• 


a 
£  ri 

|g 


X 

X 


X 
X 
X 


X 
X 


X 
X 


X 

X 


• 

X 


Remarks 


Ham.  of  New  York. 
ITp.  Held,  of  New  York. 

Up.  Held,  of  New  York,  etc. ;  Great  Bear 

Lake,  British  America. 
Ham  of  New  York. 

Devonian  of  Iowa. 

Ch.  of  New  York. 
Cn.  of  New  York. 

Up.  Held,  and  Ham.  of  New  York. 
Devonian  of  Iowa. 
Devonian  of  Iowa. 

Ch.  of  New  York. 

Up.  Held,  of  Falls  of  Ohio. 

Ham.  of  New  York. 

Of  the  type  of  P.  scmireticulatns. 

Geol.  Expl.  Fortieth  Par.,  vol.  iv,  p.  25.~>. 

Ch.  of  New  York,  Mackenzie  River,  Brit- 
ish America,  etc. 

Type,  Eureka  and  White  Pine  Districts, 
Nevada. 

Devonian  of  Iowa  and  Canada. 

Type,  Pinoii  Uungr.  Nevada. 

Up.  Held,  of  New  York,  Falls  of  Ohio,  etr. 

Geol.  Expl.  Fortieth  Par.,  vol.  iv.  p.  43. 

Up.  Held,  of  New  York,  Falls  of  Ohio.  eto. 


Up.  Held,  of  New  York  and  Canada. 

Also  in  Carboniferous. 
Ham.  of  New  York. 


Up.  Held,  and  Ham.  of  New  York. 

Ch.  of  New  York. 

Of  the  type  of  Athyris  planosulentn. 

Up.  Held,  of  New  York. 

Devonian  of  England. 

Devonian  of  America,  Europe,  etc. 

Type    from    Mackenzie    River,  British 

America. 
Ch.  of  New  York. 

Geol.  Expl.  Fortieth  Par.,  vol.  iv,  p.  217. 
Ham.  of  New  York. 


GEOLOGY  OF  THE  EUREKA  DISTRICT. 

DEVONIAN-Continued 


Genera  and  species. 


Jiracliiopoda — Continued. 
Rliynchonella  pugnus  Martin 


?  occidens  Walcott 

quadricosta  Hall 

tethys  Billings 

(L  )'lanra  Billings 

(L.)  nevadensis  Walcott. 

(L. )  siuuatus  Hall 

Leptocoelia  sp.  ? 

Pentamerus  comis  Owen  (sp.) 

lotis  Walcott 

Tropidoleptus  carinatus  Hall 

Cryptonellaf  circula  Walcott -. 

piiioucnsis  Walcott 

Terebratula,  sp.  ? 


Lamellibranchiata. 

Aviculopecten  ?  catactus  Meek 

Ptcrinopecten,  sp.  f 

Glyptodesma,  sp.  ? 

Pteriuea  newarkensis  Walcott 

rlabella  Conrad 

Actinopteria  boydi  Conrad  (sp.) 

Leiopteria  raiinesquii  Hall 

Leptodesma  trausversa  Walcott 

Limoptera  sarmenticia  Walcott 

My tilarca  dubia  Walcott 

cheuiungeusis  Conrad  (sp.) 

sp.  f 

(Plethomytilus)  oviforinis, 
Courad  (sp.). 

Modiomorpha  altiforme  Walcott 

obtnsa  Walcott 

Goniophora  perangulata  Hall 

Palii'oneilo,  sp.  ? 

Nucula  rescuensis  Walcott.-. 


sp. 


Nuculites  triangulus  H.  &  W 

iusularis  Walcott 

Megamboiiia  occidnalis  Walcott 

Ny  assa  parva  Walcott 

Grammy sia  minor  Walcott 

Edmondia  pifionensis  Meek 

iSauguiuolites?  coinbensis  Walcott 

?  gracilis  Walcott 

(Spathella)     ventricosus 
White  &  Whitfield  (sp.). 
(Spathella)  oblonga  Wal- 
cott, 

Glossitcs  ?  sandusky ensis  Meek 

Sphenotus  contractus  Hall 


Conocardium  nevadensis  Walcott 

sp 

Lunulicardium  fragosum  Meek  (sp.). 


X 
X 


X 
X 


x 
x 
x 


X 
X 


X 
X 

X 
X 
X 


X 
X 

X 


X 
X 
X 


X 
X 


X 

X 


1 


X 
X 
X 


X 
X 


X 
X 
X 


X 

X 


X 
X 


X 
X 
X 


X 
X 


Remarks. 


Devonian  of  Ohio,  New  York,  and  Eng- 
land. 

Genesee  Slate  of  New  York. 

Up.  Held,  of  Canada  and  Falls  of  Ohio. 

Ham.  of  Canada  and  New  York. 

Ch.  of  New  York. 

Not  unlike  L.  acutiplicata  Hall. 

Ham.  and  Ch.  of  Iowa. 

Found  in  Piiioii  Range,  Nevada. 


Geol.  Expl.  Fortieth  Par.,  vol.  IV,  p.  93. 


Upper  Held,  to  Ch.  of  New  York. 
Ham.  group  of  New  York. 
Ham.  group  of  New  York. 


Ch.  of  New  York. 
Ham.  of  New  York. 

Schoharie  Grit  of  New  York. 

Geol.  Expl.  Fortieth  Par.,  vol.  iv,  p.  248. 

Geol.  Expl.  Fortieth  Par.,  vol,  iv,  p.  46. 

Ch.  of  New    York;    Burlington     sand- 
stone of  Iowa. 

Up.  Held,  of  Ohio. 

Ch.   of  New   York;    Burlington    sand- 
stone of  Iowa. 

Geol.  Expl.  Fortieth  Par.,  vol.  iv,  p.  92. 


SYSTEMATIC'  LIST  OP  FOSSILS. 

DEVONIAN— Continued. 


329 


Genera  and  species. 


Lamellibranchiaia  —  Continued. 


Low 


Paracyclas  occidentalis 

peroccideiiH  H.  &  W 

Posidonomya  devonica  Walcott 

Isevis  Walcott 

Cypricardella  macrostriatus  Walcott 

Cardiomorpha  missouriensis,  Swallow  . . . 
Anadontopsis  amygdalasformis Walcott..      X 
Schizodus(Cytherodon)  orbicularis  Wai-        X 

cott. 
Cypricardinia  indenta,  Conrad  (sp. ) X 


Gasteropoda. 


Platyceras  carinatum  Hall I     X 

conicum  Hall X 


couradi  Walcott 

dentalium  Hall 

nodosum,  Conrad 

thetiforuie  Walcott . 

thetisHall 

iindulatum  Walcott . 
Platyostoma  lineatuni  Conrad. . 


X 
X 
X 
X 
X 
X 
X 


sp.? 


Euculiompbalus  devonicus  Walcott X 

Euomphalus  eurekensis  Walcott x 

(P.)  laxus  Hall 

sp.  ? X 

sp.  ? 

Sratparollus  newarkeusis  Walcott 

Plaurotomaria,  sp.  ? 

Platyschisnia  ?  ambiguum  Walcott 

?  maccoyi  Walcott 

sp.  ? 

Calonema  occidentalis  Walcott X 

Loxoueuia  approximatum  Walcott X 

eurekensis  Walcott X 

nobile  Walcott X 

?  subattenuatum  Hall X 

(2sp.  ?) X 

sp.  ? 

Bellerophon  combsi  Walcott !     X 

ledaHall '.... 

lyraHall 

ma?ra  Hall 

neleus  H.  &  W 

pelops  Hall 

perplexa  Walcott 

Scoliostoma  americana  Walcott 

Naticopsis  (like  N.  asquistriata)    

sp.? 

sp.  ? 

Metoptonia  ?  devouica  Walcott 


X 


1 


M. 


M. 
M. 


X 
X 
X 


X 


Remark*. 


Geol.  Expl.  Fortieth  Par.,  vol.  iv,  p.  248. 


Gi-ol.  Expl.  Fortieth  Par.,  vol.  iv,  p.  277. 


Upper  Held,  of  New  York  and  the  Falls 
of  the  Ohio. 


Up.  Held,  and  Ham.  of  New  York. 
Up.  Held,  and  Ham.  of  New  York,  and 
Up.  Held,  of  Falls  of  tin;  Ohio. 

Up.  Held,  of  New  York. 
Up.  Held,  of  New  York. 

Up.  Held,  and  Ham.  of  New  York. 

Up.  Held,  and  Ham.  of  New  York,  Can- 
ada, etc. 


Up.  Held,  and  Ham.  of  New  York. 


Up.  Held,  of  New  York. 


M.    ' Ham.  of  New  York. 

M.    ! Ham.' of  New  York. 

X     ! Ch.  of  New  York. 

I Geol.  Expl.  Fortieth  Par.,  vol.  iv,  p.  250. 

M Up.  Held,  of  New  York. 


v 

X 


330 


GEOLOGY  OP  THE  EUREKA  DISTRICT. 

DEVONIAN— Continued. 


Genera  and  species. 


o 

- 


I 


I 


Remarks. 


Pteropoda. 


Tentacnlites  attenuatus,  Hall 

bellulus,  Hallf 

gracilistriatus  Hall 
scalariformis  Hall . . 

Styliola  fissurella  Hall 

var.  intermittens  Hall . . 

Conularia  (sp.  f ) 

Coleolus  lievis  Walcott 

Hyolithes  (like  H.  aclis  Hall 


Cephalopoda. 

Orthoceras  (5  sp.  ?) 

Gomphoceras  suboviforme  Walcott 
Cyrtoceras  cessator,  H.  &  W 

ne vadense  Walcott 

Gouiatites  desideratus  Walcott 

kingi,H.  &W 

sp.? 


Crustacea. 

Beyriehia  occidentalis  Walcott . 
Leperditia  rotuudata  Walcott . . 


Poxilopoda. 
Phacops  rana  Green  (sp.) . . 


Dalmanites  meeki  Walcott . 

sp.  ? 

Proetus  nevadae  Hall . . 


marginalis  Conrad  (sp. ) 

sp.f  „ 

Phillipsia  coronata,  Hall  f 


X 

X 
X 

X 

x 


X 
X 


X 

X 


x 

X 


x 
x 


• 

X 


M. 


M. 


Ham.  of  New  York  and  Canada. 
Ham.  of  New  York  and  Canada. 
Ham.  of  New  York  and  Canada. 
Up.  Held,  of  New  York. 
Ham.  of  New  York  and  Canada. 


Geol.  Expl.  Fortieth  Par.,  vol.  iv,  p.  278. 


Geol.  Expl.  Fortieth  Par.,  vol.  iv.  p.  279. 
Like  (G.  discoidus  Hall). 


Up.  Held,  and  Ham.  of  New  York,  Can- 
ada, etc. 


Ham.  of  New  York,  Pal.,  N.  Y.,  vol.  vii. 

p.  129,  1888. 
Up.  Held,  of  New  York. 

Ham.  of  New  York. 


CARBONIFEROUS. 


Genera  and  species. 

Lower. 

Upper. 

Remarks. 

RHzopoda. 
Fnsilina  cylindrica  Fischer 

x 

robusta  Meek  .  . 

X 

X 

Port/era. 
Stromatopora  sp.f  

x 

Adinozoa. 
Zaphrentis  sp.  ? 

X 

Syringopora  multattenuata  ? 

X 

ChiEtetes  3  sp.  ? 

X 

SYSTEMATIC  LIST  OF  FOSSILS. 

CARBONIFEROUS— Continn.-d. 


331 


Genera  and  species. 

Lower. 

Upper. 

Remarks. 

Echinodermata. 
Arohiitocidaris  2  up.  ?  

x 

Polyzoa. 
Polypora,  sp.  ?  

x 

sp.  ?  

x 

Ptilodictya  (S.)  carbonaria  Meek?  . 

x 

(S.)  serrata  Meekf  

x 

(sp.  f  

X 

Fenestella  3sp.  ?  

x 

Brachiopoda. 
Disciua  connata  Walcott  

x 

newberryi  Hall  

x 

nitida  Phillips  (sp.  )  

x 

sp.  ?  

x 

Lingula  mytiloides  Sowerby?. 

x 

Chonetes  granulifera  Owen  . 

x 

U  S  Geol  Snrv    Nebraska  p  170  1872 

verneuiliana  N.  &  P  

x 

U.  S.  Geol.  Surv.,  Nebraska,  p.  170.  187L'. 

Productus  costatus  Sowerby  ?  

x 

Geol.  Expl.  Fortieth  Par.,  vol.  iv.,  p.69,1877. 

elegans  McCoy  

x 

longispinus  Sowerby  

x 

x 

Geol.  Expl.  Fortieth  F.ir.,  vol.  iv.,  p.  78,1877. 

longispinus,  var.  niuricatusN. 

&P. 
nebrascensis  Owen  

x 

* 

x 

Expl.  and  Surv.  W.  100th  Merid.,  vol.  'v.. 

prattenianus  Norwood    

x 

Pal.  p.  116,  1875. 
Geol.  Expl.  Fortieth  Par.,  vol,  i\  .,  p.  72,  1877. 

punctatus  Martin  (sp.)  

x 

Expl.  and   Surv.  W.  100th  Merid.,  vol.  iv, 

semireticulatus  Martin  (sp.  )  .  . 

x 

x 

Pal.  p.  114,  1875. 
Geol.  Expl.  Fortieth  Par.,  vol.  iv,  p.  69.  1S77. 

snbaculeatus  Murch 

Strophomena  rhomboidalis  Linn  (sp.) 

x 

Streptorhynchus  crenistria  Phillips  (sp.  ) 

x 

Orthis  pecosi  Marcou  

x 

Expl.  and  Surv.  W.  100th  Merid.,  Pal.,  vol. 

resupinata  Martin  (sp.  )  

x 

iv,  p.  125,  1875. 
Geol.  Expl.  Fortieth  Par.,  vol.  iv,  p.  265.  1X77. 

Spirifera  annectans  Walcott  

x 

cumerata  Morton  

x 

x 

Geol.  Expl.  Fortieth  Par.,  vol.  iv,  p.  91,  1877. 

desiderata  Walcott  

x 

leidyi  N.  &  P  

x 

neglecta  Hall     

x 

rockymontana  Marcou 

x 

x 

Expl.  and  Surv.  W.  100th  Merid.,  vol.  iv, 

striata  Martin  

x 

Pal.,  p.  134.  1875. 
Geol.  Expl.  Fortieth  Par.,  vol.iv,  p.  269.1877. 

trigonalis  Martin  (sp.) 

x 

(Martinia)  setigera  Hall 

x 

Geol.  Expl.  Fortieth  Par.,vol.  iv,  p.270.1877. 

Syringothyris  cuspidatus  Martin  (sp.) 

x 

Davidson's  Monograph.  Carb.  Brachiopoda. 

Spirifera  cristata  Schlotheim  (sp.  ) 

x 

x 

Also  in  Devonian. 

lii'  1  x  ia  radialis  Phillips  

x 

x 

veneuiliana  Hall.  .    . 

x 

L.  Carb.,  Eureka  District,  Nevada. 

Athyris  royssii  L/Eveille  (sp.  )  . 

x 

Geol.  Expl.  Fortieth  Par.,  vol.  iv,  p.  82,  1877. 

liirMita  Hall 

x 

subtilita  Hall  (sp.  )              . 

x 

x 

Rhyuchouella  enrekeusis  Walcott 

x 

thera  Walcott 

x 

sp.  f 

x 

Leiorhvnohus-like. 

Camarophoria  coopereusis  Shum    .  .  . 

x 

Terebratula  bovidens  Morton  

x 

Expl.  and  Surv.  W.  100th  Merid.,  vol.  IV, 

kastata  Sowerby  .  . 

X 

Pal.,  p.  144,  1875. 

GEOLOGY  OF  THE  EUREKA  DISTRICT. 


Systematic  list  of  fossils  of  each  geological  horizon — Continued. 
CABBONIFE  ROUS— Continued. 


Genera  and  species. 

Lower. 

Upper. 

Lamellibranchiata. 

X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 

Ha 

Ge 

Ha 

Wi 
Co 

peroccidens  Walcott  

2  sp  f 

Leptodesma  (2  sp.  ?)  -  -  .-  

Nucula  insularis  Walcott           

levatiforme  Walcott-  . 

Macrodon  hamiltona)  Hall... 

truncatus  Walcott 

teuuistriatus  Meek  &  Wortlieii  . 
Grammysia  arcuata  Conrad  (sp.)  

X 

'  XX  X  XXX  X  XXX  XX  XXX 

haunibalensis  Shnmard  (sp.). 
Edmondia  ?  circularis  Walcott  . 

medon  Walcott  . 

Pleurophorus  meeki  Walcott 

Sphenotus  seolus  Hall  .... 

retusus  Walcott 

salteri  Walcott  

simplex  Walcott  ....         ... 

Spathella  ?  u;ena  Walcott  

Cvpricardella  striata  Walcott  .  . 

connatus  Walcott 

Cardiolaf  filicostata  Walcott 

Schizodus  cuueatus  Meek 

curtifonne  Walcott 

deparcus  Walcott  

pintoensis  Walcott  

X 

X 
X 
X 
X 
X 
X 
X 
X 

X 

Gasteropoda. 
Platyceras  occidens  Walcott  .  . 

piso  Walcott  

Platyostoma  inornatum  Walcott   . 

Euomphalus  (S.)  snbrugosns  M.  &  W..  . 
Loxonema  bella  Walcott 

MacrochelluS;  sp.f 

Pleurotomaria  nevadensis  Walcott  

nodomarginata  McChes- 
ney. 

81).  f 

11  

sp.t 

X 

Remarks. 


Ham.  of  New  York. 

Geol.  Ill,  vol.  v,  p.  576, 1873. 
Ham.  of  New  York. 


Waverly  of  Ohio. 


Coal  measures  of  Ohio. 


SYSTEMATIC  LIST  OF  FOSSILS. 

Systematic  list  of  fossils  of  each  geological  horizon — Continued. 
CAKBOKIFEROU8 — Continued. 


333 


Genera  and  species. 

Lower. 

Upper. 

Remarks. 

Gasteropoda  —  Continued. 

X 

Not  unlike  N  raua  M   &  W. 

x 

X 

sp  ? 

X 

Like  B.  ellipticus  Mc.Cheevey. 

sp.  ?  

X 

....::.: 

Like  B.  sublievis  Hall. 

x 

x 



Pulmonifera. 

x 

x 

Pteropoda. 

X 

x 

x 

Cephalopoda. 

x 

x 

(2  8D  ?) 

x 

x 

Nautilus  'like  N  digonis  M  &W.) 

x 

Crustacea. 

x 

* 

Pcectlopoda. 
Griffithides  portlocki  M   &  W   (sp  ) 

x 

B. 


MICROSCOPICAL  PETROGRAPHY 

OF   THE 

ERUPTIVE  ROCKS  OF  THE  EUREKA  DISTRICT,  NEVADA. 

BY 

JOSEPH    PAXSON    IDDINGS. 


336 


APPENDIX  B. 

MICROSCOPICAL  PETROGRAPHY  OF  THE  ERUPTIVE  ROCKS  OF  THE  EUREKA 

DISTRICT,  NEVADA. 


BY  JOSEPH  PAXSON  IDDINGS. 


CHAPTEE  I. 
GRANITE  AND  PORPHYRY. 

The  representatives  of  this  division  of  eruptive  rocks  from  the  Eureka  District 
are  but  few  in  number,  ami  bear  a  very  close  resemblance  to  one  another,  being  all 
quartz- orthoclase  rocks.  They  are  composed  of  the  same  minerals,  having  in  addition 
to  the  quartz  and  orthoclase  a  triclinic  feldspar  with  biotite  and  hornblende  in  varying 
quantities.  They  are  granite,  granite-porphyry  and  quartz-porphyry. 

Granite.— Of  the  many  varieties  of  crystalline  rocks  found  within  the  small  area 
of  the  Eureka  District,  granite  plays  but  an  insignificant  role,  and  is  represented 
by  only  four  thin  sections  from  the  exposure  south  of  Ruby  Hill;  of  these,  1,  2,  and 
3  show  a  fine  grained  rock  of  uniform  texture,  with  the  characteristic  granitic  struct- 
ure. None  of  the  individuals  of  quartz  and  feldspar  have  crystallographic  outlines, 
but  are  irregularly  shaped  by  reason  of  the  mutual  penetration  of  adjacent  grains. 
The  essential  components  of  the  rock  are  quartz,  feldspar,  hornblende,  and  biotite. 
the  accessory  minerals  being  titanite,  iron  oxide,  apatite,  zircon,  and  allanite,  besides 
secondary  minerals  resulting  from  the  decomposition  of  the  first,  which  are  chlorite, 
calcite,  quartz,  epidote,  and  hydratcd  oxide  of  iron.  The  rock  is,  therefore,  an 
amphibole  granitite.  The  most  abundant  primary  constituent,  quartz,  occurs  in  irreg- 
ularly shaped  grains  which,  together  with  its  inclusion  of  portions  of  all  the  other 
primary  minerals,  shows  it  to  have  been  the  last  to  crystallize.  It  occasionally  occurs 
in  porphyritical  grains.  The  only  characteristic  inclusions  are  minute  fluid  cavities 
with  very  small  moving  bubbles.  It  shows  the  phenomena  of  irregular  optical  orien- 
tation resulting  from  mechanical  deformation.  The  feldspar  is  for  the  most  part  altered, 
but  the  fresher  sections  show  it  to  be  both  orthoclase  and  plagioclase  in  nearly  equal 
proportions.  They  both  have  a  fine  zonal  structure;  the  former  is  frequently  in  Carls- 
bad twins,  the  latter  in  multiple  twins,  after  the  albite  and  sometimes  also  after  the 
MON  xx 22  337 


338  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

pericline  law.  The  decomposition  commenced  at  the  center,  resulting  in  some  cases 
in  a  cryptocrystalline  aggregate  like  kaolin,  with  calcite;  in  others  filling  the  crystal 
with  shreds  of  colorless  mica,  and  minute,  pale  yellow  grains,  traceable  to  larger 
aggregations  of  epidote.  Feldspar  and  quartz"  form  the  main  mass  of  the  rock, 
through  which  is  scattered  mica  and  hornblende  in  varying  amounts.  The  hornblende 
is  in  poorly  defined  crystals,  except  some  of  the  smaller  individuals,  which  are  well 
developed  in  the  prism  zone.  The  prismatic  faces  are  much  larger  than  the  clinopina- 
coid,  and  the  cleavage  parallel  to  the  former  is  strongly  marked.  It  is  in  simple 
crystals  and  twins,  twinned  parallel  to  ooP  do.  The  color  is  dark  green,  with  strong 
pleochroism,  most  noticeable  in  sections  parallel  to  the  clinopinacoid  and  base.  The 
colors  are:  c  =  dark  green,  b  =  brownish  green,  a  =  light  brown,  c  =  b  >  a.  The  angle 
of  extinction  read  from  the  vertical  axis  is  mostly  from  17°  to  19°,  but  in  two  instances 
is  21°  and  25°.  It  incloses  magnetite,  apatite,  and  biotite,  having  been  formed 
after  the  latter  in  every  case.  It  is  quite  fresh,  though  the  mica  is  almost  com- 
pletely decomposed.  The  biotite,  with  which  the  hornblende  is  intimately  asso- 
ciated, occurs  in  comparatively  thick  crystals  of  irregular  outline,  of  a  deep  brown 
color,  with  nearly  uuiaxial  interference  figure,  and  has  occasional  inclusions  of  iron 
oxide,  apatite,  zircon,  and  rarely  feldspar.  It  is  especially  interesting  from  its  mode 
of  decomposition,  which  takes  place  along  the  basal  cleavage  and  results  in  a  dark 
green  pleochroic  chlorite,  which  must  be  formed  of  an  aggregation  of  -minute  scales 
parallel  to  the  lamination  of  the  mica,  for  basal  sections  remain  dark  when  revolved 
between  crossed  nicols  and  show  no  interference  figure  and  no  pleochroism,  while 
transverse  sections  exhibit  a  marked  fibration  parallel  to  the  mica  cleavage  and  are 
pleochroic ;  being  green,  parallel,  and  yellow  at  right  angles  to  the  line  of  fibration. 
This  chlorite,  in  turn,  alters  into  epidote  and  possibly  quartz.  The  epidote,  in  irregu- 
lar grains,  is  pleochroic  between  intense  greenish  yellow  and  pale  yellow.  That  it 
does  not  result  directly  from  the  decomposition  of  the  biotite  is  evident  from  the  fact 
that  it  never  occurs  in  it  unassociated  with  chlorite,  while  the  latter  occurs  constantly 
alone,  and  also  because  lenticular  masses  of  epidote  are  seen  to  have  disturbed  the  par- 
allelism of  the  chlorite  scales,  proving  its  subsequent  crystallization. 

Titanite,  in  narrow  rhombic  sections  and  less  regular  grains  is  sparingly  present. 
The  iron  oxide  appears  to  be  magnetite  for  the  most  part.  Colorless  apatite  is 
abundant  both  in  short,  stout  prisms,  and  long,  slender,  jointed  needles,  penetrating 
everything  in  all  directions.  Apatite  and  sharply  crystallized  zircon  appear  to  be  the 
first  minerals  formed  in  the  rock.  In  thin  section  2  there  are  three  comparatively 
large  crystals  of  allanite,  dark  brown,  with  strong  absorption;  two  are  twinned.  An 
irregular  grain  of  allanite  is  found  in  4.  Thin  section  3  is  highly  decomposed  and 
stained  with  hydrous  oxide  of  iron.  Thin  section  4  is  of  a  porphyritic  variety,  having 
a  fine  grained,  microgranitic  groundmass  of  quartz  and  feldspar.  Though  the  feldspar 
of  this  rock  is  still  mostly  fresh,  and  the  hornblende  entirely  so,  the  biotite  is  com- 
pletely altered  to  green  chlorite,  epidote,  quartz,  and  calcite. 


GEANITE-POEPHYltY.  339 

Granite-Porphyry.-The  microscopical  study  of  the  granite-porphyry  of  this  district, 
though  somewhat  limited,  is  of  great  interest  as  showing  the  modifications  produced 
in  the  final  crystallization  of  a  granitic  magma  through  the  chilling  caused  by  the 
inclosing  rocks,  and  consequently  the  relation  of  the  quartz  porphyries  to  the  coarse 
grained  granite;  and  also  as  pointing  out  the  correspondence  in  microscopical  structure 
of  the  metamorphosed  sandstone  of  the  district  to  certain  forms  of  micro-granite.  The 
most  important  occurrence  of  this  rock  is  in  the  granite-porphyry  dike  and  its 
apophyses  south  of  Wood  Cone,  near  Fish  Creek  Wells,  which  is  represented  by  thin 
sections  10,  11,  12,  16,  19,  20,  21,  22,  24,  25,  26,  27,  28,  29.  It  is  found  to  be  a  wholly 
crystalline  rock  of  most  varying  structure,  from  coarse  grained  granite  and  porphyritic 
granite  to  dense  porphyry  with  an  aphaiiitic  groundmass.  It  is  composed  of  quartz 
and  feldspar,  both  orthoclase  and  plagioclase,  with  a  small  amount  of  biotite  and 
hornblende;  and  since  the  character  of  these  minerals  is  the  same  throughout  the 
different  thin  sections,  and  only  their  relative  abundance  and  structural  combination 
vary,  it  seems  best  to  give  a  general  description  of  each  of  the  minerals  first,  and 
afterwards  the  special  features  which  characterize  the  different  modifications  of  the 
rock. 

The  most  noticeable  component  is  quartz,  occurring  both  in  macroscopic  pheno- 
crysts  and  in  microscopic  grains.  In  the  former  instance  it  is  usually  well  developed  in 
the  form  of  dihexahedral  crystals,  sometimes  having  short  piism  faces;  but  it  also 
occurs  in  rounded  and  irregular  grains  of  varying  size,  the  largest  being  about  as  large 
as  a  pea.  In  the  granitic  portions  of  the  rock  the  grains  are  wholly  irregular  in  form. 
The  quartz  substance  is  colorless  and  perfectly  fresh,  and  is  filled  with  minute  fluid 
inclusions,  mostly  with  a  single  gas  bubble,  sometimes  in  motion.  Frequently  there  are 
double  bubbles,  the  inner  of  which  is  sometimes  briskly  moving;  the  fluids  in  this  case 
are  water  aud  liquid  carbon  dioxide.  There  are  also  rounded  bays  of  grouudmass  pene- 
trating the  crystals,  and  more  rarely  minute  portions  of  groundmass  in  dihexahedral 
cavities.  The  habit  of  the  quartz  differs  from  that  of  the  quartz  in  quartz-porphyry 
by  the  abundance  of  liquid  carbon  dioxide  and  the  absence  of  any  isotropic  glass,  but 
corresponds  closely  to  it  in  other  respects.  The  microscopic  grains  of  quartz  which 
form  a  large  part  of  the  groundmass,  have  a  granitic  habit,  being  in  part  irregularly 
outlined,  in  part  conjointly  crystallized  with  the  feldspar,  producing  micropegmatitic 
structure,  to  be  described  later  on. 

The  feldspar  is  mostly  orthoclase,  but  a  triclinic  species  also  is  always  present, 
the  large  phenocrysts  of  the  former  are  often  well  crystallized  with  the  ordinary  faces, 
ooP^,  OP,  »P,2Pa>,  the  cliuopinacoid  being  the  most  strongly  developed,  forming 
tabular  Carlsbad  twins.  The  cleavage  parallel  to  the  base  is  very  perfect,  that  paral- 
lel to  the  cliuopinacoid  less  so,  and  in  numerous  individuals  a  fine  striping  is  noticed, 
which  is  remarkably  regular,  but  occasionally  deviates  from  right  lines  and  loses  its 
parallelism.  It  at  first  suggests  the  polysynthetic  twinning  of  plagioclase,  but  on 
closer  examination  appears  to  be  an  iuterla  mi  nation  of  albite  iu  orthoclase  parallel  to 


340  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

the  orthoptnacoid,  as  in  perthite;  iu  one  section  nearly  in  the  plane  of  the  clinopina- 
coid  the  striping  crosses  the  basal  cleavage  parallel  to  the  direction  of  the  other 
piuacoid,  and  the  angles  of  extinction  for  the  main  crystal  and  the  included  lamellae 
are  about  7°  and  18°,  respectively,  on  the  same  side  of  the  basal  cleavage ;  angles  which 
correspond  to  orthoclase  and  albite  in  such  a  section.  A  zonal  structure  is  com- 
mon to  many  individuals,  and  may  be  observed  in  the  fresher  crystals,  even  in  the 
hand  specimens,  without  the  aid  of  a  lens.  There  are  no  characteristic  inclusions,  „ 
but  particles  of  the  associated  minerals  are  frequently  met  with,  especially  near  the 
margin  of  the  crystal.  The  plagioclase  is  very  similar  iu  occurrence  to  the  orthoclase, 
being  characterized  by  the  abundance  of  striations  produced  by  multiple  twinning, 
mostly  in  one  direction,  like  that  in  albite,  but  also  in  a  second  direction  nearly  at  right 
angles  to  the  first,  like  that  in  pericline.  Exactly  how  many  species  are  present  has 
not  been  determined  optically,  but  it  is  certain  that  labradorite  is  one  of  them,  as  the 
highest  symmetrical  extinction  angles  reach  about  30°.  It  is  in  general  quite  free 
from  inclusions;  nevertheless,  in  some  of  the  plagioclase  crystals  from  widely  different 
parts  of  the  dike  there  are  minute,  colorless,  rectangular  bodies  always  parallel  to 
the  twinned  lamell<e,  that  at  once  remind  one  of  the  glass  inclusions  characteristic  of 
andesitic  plagioclase.  Their  nature,  however,  is  doubtful,  for  though  without  influence 
on  polarized  light  in  most  instances,  they  appear  in  others  to  affect  it  slightly,  and 
besides  are  without  a  gas-bubble,  from  which  it  seems  probable  that  they  are  not 
glass,  but  possibly  feldspar.  The  substance  of  the  feldspar  is  sometimes  perfectly 
fresh  and  transparent,  at  others  clouded  by  minute,  irregularly  shaped  particles,  that 
reflect  incident  light  and  appear  white.  The  orthoclase  when  further  altered  is  filled 
with  brilliantly  polarizing  shreds  of  colorless  potash-mica,  arranged  parallel  to  three 
directions  in  the  crystal.  Calcite  is  noticed  iu  the  partially  decomposed  plagioclase, 
the  decomposition  in  general  setting  in  from  the  outside  of  a  crystal  and  traversing  it 
in  the  most  irregular  manner. 

The  next  essential  mineral  to  be  mentioned  is  biotite.  It  is  universally  present, 
but  in  varying  quantities.  In  the  thin  sections  from  this  body  it  is  mostly  altered. 
The  fresh  mineral  is  in  poorly  denned,  six-sided  crystals  of  a  dark  brown  color, 
with  strong  absorption  parallel  to  the  basal  cleavage,  the  basal  sections  yielding 
apparently  uniaxial  interference  figures,  with  a  negative  character,  but  sometimes 
showing  a  small  angle  between  the  optic  axes.  It  is  quite  free  from  inclusions,  but 
occasionally  carries  small  crystals  of  zircon  and  apatite,  and  more  frequently  titanic 
iron;  in  one  instance  (12)  it  is  surrounded  by  grains  of  iron  oxide,  and  in  another  (22) 
by  hornblende.  The  alteration  that  has  taken  place  iu  most  of  the  sections  appears 
to  be  a  bleaching  out  of  the  brown  color,  leaving  a  yellow  or  light  green,  brilliantly 
polarizing  mica,  with  faint  pleochroism,  which  is  generally  filled  with  slender  acicular 
crystals  of  a  yellowish  brown  color  arranged  in  Hues  intersecting  at  60°,  besides 
larger  and  stouter  crystals  very  perfectly  developed,  which  have  a  high  index  of 
refraction  and  seem  to  belong  to  the  tetragonal  system.  From  their  close  association 


GRANITE-PORPHYRY.  341 

with  partially  decomposed  titanic  iron,  which,  is  characterized  by  strongly  marked, 
rhombohedral  cleavage,  it  is  most  likely  that  these  minute,  secondary  crystals  are 
rutile  or  anatase.  The  decomposition  starts  from  the  surface  of  the  crystal,  sections 
of  partially  altered  mica  being  found  with  portions  of  the  mineral  still  fresh  in  the 
center.  A  further  stage  of  alteration  produces  a  green,  fibrous  chlorite,  and  in  one 
instance  (12)  quartz  and  epidote.  The  colorless  potash-mica,  scattered  through  the 
groundmass  in  shreds  and  fan-like  aggregations,  which  appears  brilliantly  colored 
between  crossed  nicols,  and  shows  a  small  angle  between  the  optic  axes,  is  undoubt- 
edly of  secondary  origin,  arising  from  the  decomposition  of  orthoclase,  as  already 
mentioned,  or  from  that  of  the  brown  mica ;  for  in  every  thin  section  where  it  occurs 
both  the  brown  mica  and  feldspar  are  more  or  less  altered,  and  in  those  where  they 
are  both  perfectly  fresh  it  is  wanting. 

The  hornblende  is  by  no  means  a  constant  ingredient,  being  absent  from  all  the 
more  porphyry-like  varieties  and  present  in  only  part  of  the  granitic  ones.  It  is  of  a 
dark  brown  color,  sometimes  green,  with  strong  absorption  and  pleochroism,  and  is 
seldom  in  fully  developed  crystals,  though  some  cross  sections  with  the  characteristic 
cleavage  show  the  presence  of  the  prism  and  both  the  pinacoidal  faces.  The  crystals 
are  short  and  stout,  their  outline  broken  by  intruding  grains  of  the  surrounding 
groundmass,  which  are  also  abundantly  included  in  the  hornblende,  together  with 
apatite,  iron  oxide,  and  more  rarely  mica.  It  would  seem  to  be  one  of  the  later  crys- 
tallizations, contemporaneous  with  that  of  the  ground  mass. 

There  are  a  great  number  of  accessory  minerals,  which  are  not  all  present, 
however,  in  any  one  thin  section,  and  are  more  abundant  in  the  granitic  than  in  the 
porphyry-like  forms  of  the  rock.  The  most  exceptional  of  these  is  augite,  found  in 
only  one  thin  section  (22).  It  is  of  pale  green  color,  with  high  index  of  refraction  and 
characteristically  great  extinction  angle.  Titanite  or  sphene  in  wedge-shaped  crystals 
and  irregular  grains  is  common  to  the  hornblende-bearing  varieties,  with  which  min- 
eral it  is  usually  in  close  association.  The  iron  oxide  appears  to  be  for  the  most  part 
titaniferous,  many  of  the  larger  grains  showing  a  most  pronounced  rhombohedral 
cleavage,  the  decomposition  in  several  cases  resulting  in  lencoxene  and  a  chemical 
test  giving  the  reaction  for  titanic  acid.  Apatite  and  zircon  in  sharply  defined 
crystals  are  everywhere  present  in  small  quantities,  and  garnet  is  found  in  a  thin  sec- 
tion from  a  closely  related  dike.  Allanite  is  present  in  those  sections  rich  in  hornblende 
and  biotite;  it  is  especially  abundant  in  No.  11,  where  ten  grains  of  it  were  noted. 

The  groundmass  of  this  rock,  in  all  of  its  modifications,  is  wholly  crystalline, 
no  isotropic  glass  being  found  anywhere  in  it.  It  is  composed  of  quartz  and  ortho- 
clase feldspar,  very  little  plagioclase  having  been  recognized.  To  these  is  sometimes 
added  hornblende,  biotite,  and  titanic  iron,  besides  the  colorless  potash -mu-a  of 
secondary  origin.  The  quartz  and  feldspar  are  either  in  an  aggregation  of  irregularly 
outlined  grains  of  nearly  uniform  size,  which  is  the  ordinary  structnie  of  granite,  or 
they  form  orderly  arranged  groups  of  triangular  or  rhombic  figures,  and  others  clou- 


342  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

prated  and  feather-like.  This  structure  is  noticeable  in  ordinary  light  from  the  fact 
that  the  quartz  remains  pellucid  after  the  feldspar  has  become  clouded  by  partial 
alteration.  Between  crossed  nicols,  however,  the  appearance  is  very  distinct.  In  the 
coarser  grained  varieties,  especially  at  the  spot  represented  in  Fig.  1,  PL  vi,  the 
field  is  covered  with  blocks  of  similar  geometric  and  cuneiform  figures — parallel- 
ograms, trapezoids,  and  variously  shaped  triangles — the  sides  of  all  those  forming  any 
single  group  being  respectively  parallel,  besides  which  are  long  and  narrow  parallel 
strips,  two  sets  of  which,  meeting  obliquely,  produce  a  feather  like  appearance.  It  is 
further  seen  that  all  the  figures  in  any  one  group  extinguish  light  in  the  same  azimuth 
or  have  the  same  optical  orientation,  and  that  the  inclosing  block  is  a  single  individual 
with  a  different  orientation,  the  one  being  quartz  and  the  other  feldspar.  In  this  par- 
ticular case  the  small  figures  are  of  quartz  and  throughout  the  field  have  the  same 
extinction  as  a  central  grain,  the  inclosing  blocks  being  of  differently  oriented  feld- 
spar. This  tendency  to  crystallize  around  grains  of  quartz  or  feldspar  is  more 
noticeable  in  the  finer  grained  varieties,  where  the  nucleus  is  incrusted  with  a  shell 
that  in  section  appears  as  a  frame-like  border,  having  a  radiating  structure  composed 
of  variously  oriented  sectors,  though  the  portions  formed  by  the  same  mineral  as  the 
nucleus  have  their  axes  of  elasticity  parallel  throughout.  Of  the  phenocrysts,  quartz 
seems  to  be  the  only  one  around  which  this  special  crystallization  takes  place.1 
A  very  fine  example  of  this  structure  is  found  in  thin  section  28,  where  it  is  seen 
to  have  formed  after  the  primary  crystallization  of  the  phenocrysts,  but  previous  to 
the  final  consolidation  of  the  micrograuitic  groundmass.  This  distinctive  structure, 
which  is  characteristic  of  many  European  granite-porphyries,  has  been  described 
by  Eosenbusch  and  called  by  him  "Granophyr."2  It  is  the  "structure  pegmatoide" 
of  the  French  petrographers,  and  is  becoming  generally  termed  micropeginatitic. 

Having  described  the  characters  of  the  minerals  composing  this  granite- 
porphyry,  it  remains  to  notice  their  structural  combination,  whose  variety  is  the  strik- 
ing feature  of  this  occurrence.  Thin  section  10,  from  the  large  area  north  of  Wood 
Cone,  is  of  a  porphyritic  granite,  with  little  groundmass  of  fine  grained  granitoid 
structure.  The  large  phenocrysts  have  no  crystallographic  outline,  but  pass  by 
increasing  abundance  of  inclusions  into  the  groundmass,  which  contains  biotite, 
hornblende,  titanite  and  titanic  iron,  apatite,  zircon,  and  allanite.  Thin  section  11, 
from  southwest  of  the  Wood  Cone,  is  a  local  modification  of  slight  importance;  it  is 
a  fine  grained  mass  without  phenocrysts,  with  granitoid  structure  and  composed  of  the 
same  minerals  as  the  previous  section,  but  with  a  greater  percentage  of  biotite  and 
hornblende.  Thin  section  12,  from  the  bottom  of  a  gulch  on  the  east  side  of  the  dike 
and  north  of  Spring  Valley  road,  is  of  porphyritic,  coarse  grained  granite.  The 
larger  phenocrysts  of  feldspar  have  more  or  less  well  defined  outlines.  The  orthoclases 


'This  constitutes  the  quartz  aureole  of  French  petrographers. 

!H.  Rosenbusch.   Die  Steiger  Schiefer,  etc.,  pp.  347,  352,  Strassburg,  1877.   Mikroskopisehe  Physi- 
ograpie.  p.  31,  Stuttgart,  1877.     Mikroskopisehe  Physiograpie,  vol.  n,  p.  383,  Stuttgart,  1886. 


GRANITE  PORPHYRY.  343 

have  a  marginal  zone  of  included  quartz  grains,  the  inner  limit  of  the  zone  being 
sharply  defined.  In  places  the  inclosed  grains  are  so  numerous  that  the  feldspar 
crystals  merge  in  the  groundmass  and  their  outline  is  confused.  The  biotite,  sur- 
rounded by  grains  of  iron  oxide,  is  partly  altered  to  green  chlorite  and  epidote.  The 
hornblende  is  scarce  and  the  crystallization  of  the  groundmass  granitoid.  The  next 
three  sections — 16,  19,  20 — should  be  considered  together,  since  they  are  from  the 
same  portion  of  the  west  side  of  the  dike  north  of  the  last  named  road.  No.  16  is 
from  a  distance  of  30  feet  from  the  plane  of  contact  with  the  limestone  and  is  rich  in 
sharply  defined  porphyritical  crystals,  the  quartz  being  in  perfect  dihexahedrons. 
There  is  an  abundance  of  biotite  and  titanic  iron  with  titanite,  but  no  hornblende. 
The  groundmass  has  the  coarse  grained  micrOpegrnatitic  structure  illustrated  in  Fig. 
1,  PI.  vi.  No.  19,  from  a  distance  of  10  feet,  and  No.  20,  from  the  contact,  are  still 
more  porphyry-like,  having  much  more  groundmass  with  finer  grained  micropeg- 
matitic  structure,  of  very  homogeneous  texture,  the  only  phenocrysts  being  feldspar 
and  quartz;  oiotite,  hornblende,  and  the  associated  minerals  are  wanting.  The  feld- 
spar is  more  altered  than  that  of  16  and  colorless  potash-mica  is  more  abundant. 
Thin  sections  21  and  22  are  from  the  bottom  of  a  gulch  on  the  south  side  of  the  same 
road.  The  first  from  a  distance  of  1  foot  from  the  contact  with  limestone,  shows  a 
porphyritic  granite,  rich  in  large,  well  defined  crystals  of  feldspar  and  quartz,  with 
much  biotite  and  hornblende.  The  groundmass  forms  but  a  small  part  of  the  whole 
and  is  granitoid,  the  grains  averaging  0-1 mm  in  size.  The  second  thin  section  is  from 
immediate  contact  and  differs  from  the  first  in  having  much  more  groundmass  with  the 
same  microgranular  structure,  the  grains  being  only  one- third  as  large  as  those  at  a 
foot  distance.  The  phenocrysts  are  smaller  and  more  sharply  outlined,  the  fresh 
orthoclase  having  a  satin-like  sheen  in  thin  section.  Hornblende  is  more  abundant  in 
minute  crystals,  and  augite  occurs  sparingly. 

Thin  section  24,  from  a  local  modification  of  the  porphyry  near  its  contact  with 
limestone  on  the  north  side  of  the  road,  requires  special  notice.  The  rather  small 
phenocrysts  of  quartz  and  feldspar  have  the  same  characteristics  as  those  from  other 
portions  of  the  same  body,  but  the  groundmass  is  very  diiferent.  It  is  bluish  gray  iu 
thin  section,  spotted  with  minute  black  specks;  under  the  microscope  it  is  seen  to  be 
composed  of  irregular  grains  of  quartz  and  feldspar  having  a  granitic  structure.  The 
black  specks  are  found  to  be  angular  microcrystalline  patches,  crowded  with  black 
particles,  and  bearing  shreds  of  colorless  uu'ca,  and  appear  to  be  remnants  of  a  base  less 
highly  crystallized  than  the  groundmass.  Thin  section  25  is  from  a  breccia  of  por- 
phyry and  dark  colored  quartzite,  the  phenocrysts  are  angular  fragments,  the  quartz 
is  rich  in  fluid  inclusions,  and  the  opaque,  black  portions  produce  a  very  prominent 
flow-structure.  Both  sections  are  free  from  biotite  or  hornblende.  Thin  sections  26  aud 
27  are  interesting  because  they  come  from  the  middle  and  side  of  a  branch  dike 
not  30  foot  wide.  The  former  has  a  dense  greundmass,  bearing  large  quartz  dihexahe- 


344  GEOLOGY  OF  THE  ETTBEKA  DISTRICT. 

drons  fairly  flooded  with  fluid  inclusions  of  water  and  liquid  carbon  dioxide;  the 
groundmass  is  a  comparatively  coarse  grained  aggregation  of  quartz  and  feldspar, 
the  latter  more  highly  developed  but  entirely  decomposed,  there  is  a  little  completely 
altered  biotite,  much  colorless  mica  and  some  epidote ;  the  second  thin  section,  from 
the  side  of  the  dike,  is  a  dense  gray  mass,  poor  in  quartz  pheuocrysts,  but  rich  in 
small  crystals  of  feldspar  and  biotite,  the  latter  partially  altered  to  chlorite;  the 
groundmass  is  finer  grained  and  shows  in  places  an  incipient  micropegmatitic  struc- 
ture, which  is  noticeable  around  the  quartz  crystals  and  also  in  pseudospherulites ; 
there  is  a  little  titanite,  but  no  hornblende. 

Thin  section  28,  from  a  narrow  dike  farther  up  the  Spring  Valley  road,  has 
been  already  alluded  to  as  presenting  a  most  beautiful  example  of  micropegmatitic 
structure.  It  is  a  very  fine  grained  rock,  having  a  few  small  quartz  dihexahedrons 
containing  fluid  inclusions  of  both  kinds  and  portions  of  groundmass,  besides  a 
few  crystals  of  feldspar,  but  no  biotite  or  hornblende.  Around  the  quartz  and 
smallest  feldspars  are  frames  of  feather  or  fernlike  aggregates  of  intercrystallized 
feldspar  and  quartz,  producing  the  effect  of  a  flowered  pattern  on  the  microgranitic 
groundmass  of  quartz,  feldspar  and  colorless  mica.  Thin  section  29  from  a  small  dike 
west  of  Castle  Mountain  is  somewhat  similar;  the  extremely  fine  grained  groundmass 
bears  numerous  quartz  crystals,  with  bays  of  groundmass,  some  inclusions  of  color, 
less  glass  and  a  few  of  water  with  moving  bubbles,  also  crystals  of  feldspar  com- 
pletely altered  to  a  cryptocrystalline  aggregate,  probably  kaolin,  besides  calcite  and 
hydrous  oxide  of  iron;  and  a  little  decomposed  biotite.  The  groundmass  is  micro- 
granitic  with  an  incipient,  micropegmatitic  structure  developed  around  the  quartzes. 
Garnet  occurs  iu  well  formed  rhombic  dodecahedrons,  having  long  slender  needles 
radiating  from  their  centers,  which  exert  no  influence  on  polarized  light  and  are  of 
an  indeterminable  nature. 

Another  variety  of  granite-porphyry  is  found  near  the  summit  of  the  Fish 
Creek  Mountains,  thin  section  30.  It  is  a  fine  grained  rock,  rich  in  biotite  and  horn- 
blende. TL  e  sections  show  it  to  be  composed  of  long  rectangular  feldspar  crystals, 
six-sided  mica  plates  and  rather  stout  hornblende  crystals  cemented  together  by 
quartz  and  feldspar,  with  well  developed  micropegmatitic  structure.  Except  in  this 
last  respect  the  rock  closely  resembles  the  fine  grained  micaceous  modification  of  the 
large  granite  porphyry  dike  near  Wood  Cone,  No.  11.  The  feldspar  is  much  altered, 
chiefly  at  the  center  of  each  individual,  the  product  being  partly  potash-mica,  partly 
calcite.  Of  the  fresher  crystals  many  are  triclinic.  An  estimate  of  the  relative 
abundance  of  the  two  feldspars,  however,  is  impossible  under  the  circumstances. 
Quartz  does  not  occur  iu  large  pheuocrysts,  but  forms  the  greater  part  of  the  ground- 
mass,  where,  with  a  little  feldspar,  it  assumes  the  peculiar  structure  already  alluded 
to.  It  carries  a  small  number  of  minute  fluid  inclusions  with  moving  bubbles.  The 
biotite,  in  quite  sharply  outlined  six-sided  crystals,  is  reddish  brown  in  its  fresher  por- 


QUABTZ-POEPHYEY.  345 

tions  and  shows  a  slight  angle  between  the  optic  axes,  but  it  is  mostly  altered  to  a 
light  yellow  chlorite,  through  which  are  scattered  grains  of  a  yellow,  highly  refracting 
mineral,  resulting  from  the  alteration  of  ilmenite  and  corresponding  to  leucoxene, 
besides  very  small,  sharply  defined,  colorless  crystals,  apparently  epidote.  The 
crystals  of  hornblende  are  very  well  developed,  showing  the  prismatic  and  both 
pinacoidal  faces,  together  with  the  base  and  pyramid.  The  individuals  are  compara- 
tively large  and  broad,  with  the  characteristic  cleavage.  The  color  is  light  brown, 
frequently  green  along  the  margin.  The  pleochroism  is  strong  from  brown  to  yellow, 
c=  b>  a.  The  highest  extinction  angle  measures  19°.  Along  the  cleavage  crack  red 
oxide  of  iron  is  sometimes  deposited,  and,  though  for  the  most  part  fresh,  a  few  are 
completely  altered  to  an  irregular  aggregate  of  fibrous  chlorite  and  hydrous  oxide  of 
iron,  through  which  run  colorless  needles  with  an  extinction  angle  of  17°,  which  are 
probably  actinolite.  The  accessory  minerals  are  magnetite,  with  some  ilmenite  partly 
altered  to  leucoxene,  a  very  little  titanite,  and  a  large  amount  of  apatite,  both  in 
short  crystals  and  also  in  extremely  long,  slender,  colorless,  hexagonal  prisms,  occa- 
sionally broken  and  bent,  but  generally  perfectly  straight,  although  one  measures 
0-44  mm  long  by  0-0075  mm  wide,  or  is  sixty  times  as  long  as  it  is  broad,  which  indicates 
that  the  mass  commenced  to  crystallize  after  all  motion  in  it  had  ceased. 

Quaru-porphyry.-Unfortuuately  the  only  body  of  quartz-porphyry  found  in  the  dis- 
trict is  completely  decomposed.  It  occurs  in  the  vicinity  of  the  Bullwhacker  mine 
and  is  represented  by  thin  sections  31,  32,  and  33,  which  have  essentially  the  same 
structure,  though  the  first  is  full  of  pyrite  and  the  second  and  third  are  discolored  by 
hydrous  oxide  of  iron.  It  is  closely  related  to  granite-porphyry,  having  apparently  a 
microgranitic  groundmass;  but  a  thin  film  of  isotropic  glass  is  detected  between  the 
grains  along  the  thinnest  edge  of  section  31,  and  colorless  glass  is  found  included  in 
the  macroscopic  quartz  grains,  whose  quartz-porphyry  habit  is  further  evinced  by  intru- 
sions of  groundmass,  small  amount  of  fluid  inclusions,  some  of  which  have  salt  cubes, 
and  by  the  absence  of  liquid  carbon  dioxide.  The  quartz  shows  a  well  developed 
rhombohedral  cleavage,  especially  in  section  32,  and  is  the  only  primary  mineral  except 
apatite  and  zircon  remaining  unaltered.  A  small  amount  of  feldspar  is  indicated  by 
patches  of  a  colorless,  aggregately  polarizing  substance,  probably  kaolin.  The  mica 
occurs  in  comparatively  large  crystals,  much  elongated  in  the  direction  of  the  vertical 
axis,  which  have  been  altered  to  a  mass  of  confused  lamina1  of  colorless  potash-mica, 
calcite  and  red  oxide  of  iron.  The  groundmass  also  is  crowded  with  shreds  of  potash- 
mica,  but  it  seems  probable  that  in  both  of  its  occurrences  it  is  of  secondary  origin. 
Sections  that  have  the  outline  of  hornblende  crystals  are  filled  with  calcite  and  ferrite, 
and  quite  large  deposits  of  calcite  with  very  distinct  rhombohedral  cleavage  have  tilled 
cavities  in  the  rock.  Iron  is  present  as  magnetite  and  the  hydrous  oxides  and  as 
ilmenite  and  pyrite,  the  latter  in  comparatively  large  crystals,  including  portions  of 
the  groundmass.  Apatite  and  zircon  occur  in  very  small  quantities. 


346  GEOLOGY  OF  THE  ETJEEKA  DISTEICT. 

Appendix— Metamorphosed  Sandstone— A  micaceous  fine  grained  rock  occurs  in  several 
localities  in  the  vicinity  of  Modoc  Peak,  which  is  traceable  to  thin  beds  of  sandstone, 
which,  however,  are  never  so  full  of  mica,  and  though  the  true  nature  of  its  occurrence 
is  somewhat  in  doubt  it  is  safe  to  consider  it  a  highly  altered  forms  of  the  same 
quartzose  deposit,  since  a  series  of  thin  sections  from  the  bedded  sandstone  and  the 
very  micaceous  rock  grades  imperceptibly  from  one  extreme  to  the  other,  the  coarsest 
grained  variety  having  the  mineral  composition  and  structure  of  a  microgranite.  Of 
the  thin  sections  prepared,  three,  437,  440, 450,  are  from  dense  cryptocrystalliiie  sand- 
stone of  a  yellowish  pink  color,  bearing  a  few  quite  perfect  crystals  of  muscovite  and 
quartz.  Under  the  microscope  the  rock  is  seen  to  be  formed  of  minute  quartz  grains, 
shreds  of  potash-mica  and  patches  of  a  colorless  cryptocrystalliue  substance,  through 
all  of  which  is  scattered  much  calcite  and  ferrite,  and  occasionally  long,  slender,  beau- 
tifully terminated  crystals  of  zircon.  The  quartz  grains  range  from  0-1  to  0-05mm  in 
diameter,  and  have  the  granitoid  form,  in  no  instance  suggesting  waterworu  fragments. 
They  contain  extremely  minute  fluid  inclusions,  which  literally  swarm  in  the  micro- 
scopic quartz  dihexahedrons  of  section  440.  The  form  in  which  the  calcite  is  found 
suggests  its  alteration  from  feldspar,  or  its  deposition  by  infiltration  in  the  place  of 
decomposed  feldspar,  which  is  undoubtedly  the  case  in  one  or  two  quite  large  sections. 
Thin  section  451  is  very  similar  to  those  just  mentioned,  and  with  452  came  from  a  body 
closely  connected  with  the  bed  of  sandstone  represented  by  450.  The  grouudmass  of 
451  is  the  same  in  every  respect  as  those  just  described,  but  there  are  numerous 
macroscopical  individuals  of  mica  and  feldspar,  the  latter  still  showing  in  some  cases 
the  striping  of  twinned  plagioclase,  though  mostly  altered  to  a  cryptocrystalline  mass 
like  that  occurring  in  the  groundmass,  which  may  probably  have  the  same  origin. 
The  poorly  denned  mica  is  completely  replaced  by  calcite  and  ferrite.  Sharply  crys- 
tallized zircon  and  apatite  are  present  in  stout  prisms  with  very  uneven  outline. 
The  three  remaining  thin  sections,  452,  466,  and  463,  exhibit  the  highest  development 
reached,  and  might  be  considered  micaceous  microgranite.  The  groundmass  is  com- 
posed of  granitoid  quartz  and  partially  altered  feldspar  in  grains  about  0-lmm  in  diame- 
ter together  with  shreds  of  potash-mica  and  calcite.  In  this  lie  porphyritically  imbed- 
ded well  developed  feldspar  crystals  and  mica  and  occasionally  quartz.  The  feldspar 
is  much  altered,  but  shows  that  it  is  partly  Carlsbad  twins,  and  is  partly  striped 
plagioclase.  The  mica  is  somewhat  altered  and  is  of  brownish  yellow  color,  with 
strong  absorption.  A  large  individual  of  quartz  in  466  has  a  multitude  of  minute 
fluid  inclusions  arranged  in  planes  parallel  to  the  prism  or  rhombohedral  faces.  Ferrite, 
apatite  and  zircon  also  occur.  The  whole  is  a  thoroughly  granite-like  rock,  without 
signs  of  foliation. 

It  is  interesting  to  note  in  this  connection  how  an  apparent  granitoid  form  of 
quartz  grains  may  sometimes  arise  from  an  entirely  different  cause.  The  phenomenon 
is  exhibited  in  a  quartz  conglomerate  of  small  grain,  thin  section  501,  where  it  is 


METAMORPHOSED  SANDSTONE.  347 

observed  in  polarized  light  that  between  the  coarser  waterworn  fragments  of  colored, 
cryptocry stall iue  quartzite  lies  a  mass  of  colorless  quartz  in  angular,  closely  fitting 
grains,  the  salient  angles  of  one  corresponding  to  reentrant  angles  of  those  surround- 
ing it.  Upon  close  examination  in  ordinary  light  each  angular  crystal  is  seen  to 
inclose  a  large  round  grain  of  quartz,  frequently  full  of  fluid  inclusions  and  contain- 
ing inicrolites  and  trichites,  the  narrow  border  being  perfectly  pure  quartz.  This 
is  illustrated  in  Fig.  3,  PI.  iv.  From  this  it  is  evident  that  the  rock  is  an  ordinary 
quartz  conglomerate  of  rounded  pebbles  cemented  together  by  silica  that  has  crystal- 
lized around  the  fragments  of  quartz  crystals,  taking  the  same  crystallographic  orien- 
tation as  the  nucleus  and  thus  extending  the  individual  until  obstructed  by  the 
surrounding  bodies. 

The  same  observations  were  first  made  and  published  by  Tornebohni1  in  1876 
and  subsequently  were  observed  by  H.  Clifton  Sorby2  and  published  by  him  in  an 
address  before  the  Geological  Society  of  London,  February  20,  1880.  The  same  phe- 
nomenon was  described  by  A.  A.  Young  in  the  American  Journal  of  Science  for  July, 
1881;  and  still  later,  in  1883,  R.  D.  Irving  published  in  the  same  journal  for  June 
a  paper  on  the  similar  enlargement  of  quartz  grains  in  the  St.  Peters  and  Potsdam 
sandstones  and  in  certain  Archean  quartzites  in  Wisconsin,  and  in  1884  Irving  and 
"Van  Hise published  a  bulletin  "on  secondary  enlargements  of  mineral  fragments  in 
certain  rocks," 3  in  which,  in  addition  to  quartz,  the  enlargement  of  feldspars  by  the 
same  process  of  accretionary  crystallization  is  described. 

The  same  thing  has  been  observed  by  T.  G.  Bonney  and  Mr.  J.  A.  Phillips  in 
England.4 

'A.  E   Tornebohm,  "Ein  Beitrag  zur  Frage  der  Qnarzitbildung."    Geol.  Foren  Stockh,  1876,  vol. 
Ill,  p.  35.     Reviewed  in  Neues  Jahrbuch  fur  Min.,  etc.,  1877,  p.  210. 
'Quart.  Journ.  Geol.  Soc.,  London,  1880,  vol.  xxxvi,  p.  62. 
3  Bull.  8  of  the  U.  S.  Geol.  Survey,  1884. 
••Quart.  Journ.  Geol.  Soc.,  London,  vol.  xxxix,  p.  19. 


CHAPTER   II. 
VOLCANIC  ROCKS. 

For  so  small  an  area  the  variety  of  volcanic  rocks  is  great,  yet  there  is  a 
marked  similarity  between  the  individual  crystals  of  the  same  mineral  species 
wherever  they  occur,  with  some  few  exceptions,  which  links  the  various  kinds  of  rocks 
together  and  suggests  the  possibility  of  a  common  source.  Nevertheless,  the  differ- 
ence between  them  in  composition,  structure,  and  physical  appearance  is  sufficient 
to  establish  their  individuality.  They  have  been  divided  into  three  groups — anclesite, 
rhyolite,  and  basalt — and  have  been  considered  in  the  order  of  their  relative  impor- 
tance in  the  field. 

ANDESITE. 

pyroxene-andesite  (augite-andesite).  —(a.)1  The  rock  forming  Richmond  Mountain  isa  dense 
porphyritic  lava,  for  the  most  part  with  a  reddish  purple  homogeneous  groundmass, 
rich  in  macroscopic  crystals  of  flesh  colored  feldspar,  the  largest  4  or  5"""  long,  without 
distinct  cleavage,  and  having  a  inierotine  habit;  long  black  prisms  of  hornblende,  with 
very  perfect  prismatic  cleavage  and  less  noticeable  pyroxene  crystals.  The  dense 
purple  variety  is  in  most  every  case  parted  or  jointed  in  nearly  horizontal  planes. 
A  dark  bluish  black  variety,  with  a  resinous  oily  luster,  occurs  in  compact  masses 
without  fissile  structure,  and  appears  to  pass  insensibly  into  the  purple  rock.  At 
Trail  Hill,  the  most  northern  spur  of  Richmond  Mountain,  the  same  rock  traced 
continuously  from  the  main  portion  is  vesicular  and  is  rich  in  triclymite.  A  few 
hundred  yards  to  the  south  a  compact  fissile  exposure  shows  a  more  crystalline 
development  and  is  exceptional. 

Under  the  microscope  thin  sections  from  various  parts  of  the  body  have  essen- 
tially the  same  character — a  gray,  also  yellowish  to  reddish  gray,  grouudmass,  com- 
posed of  colorless  or  yellowish  brown  glass,  very  rich  in  feldspar  microlites,  augite 
prisms,  and  magnetite  grains,  with  marked  flow  structure;  abundant  phenocrysts  of 
zonally  built  plagioclase  feldspar,  with  and  without  polysyuthetic  twinning,  the 


1  Since  the  first  determination  of  these  rocks  was  made,  a  separation  and  optical  and  chemical 
analysis  of  the  pyroxenic  constituent  of  the  Richmond  Mountain  anaesite  have  been  made  and  pub- 
lished in  the  "Notes  on  the  volcanic  rocks  of  the  Great  Basin,"  by  Arnold  Hague  and  J.  P.  Iddings 
(Am.  Journ.  Sci.,  Vol.  xxvn,  June,  1884,  p.  458).  This  showed  that  the  greater  portion  of  the  pyox- 
ene  belongs  to  the  orthorhombic  species  and  has  the  composition  of  hyperstheue.  It  is  therefore 
more  correct  to  place  them  under  the  head  of  pyroxene-andesites,  though  they  were  first  termed 
"augite"  andesites,  those  from  Richmond  Mountain  belonging  to  the  hornblende-bearing  variety, 
348 


PTEOXENE-ANDESITE.  349 

largest  of  which  are  so  crowded  with  inclusions  of  foreign  matter,  with  only  a  narrow 
border  of  pure  feldspar,  as  to  appear  decomposed  in  the  hand  specimen.  The  smaller 
individuals  of  feldspar  are  quite  free  from  like  inclusions.  Besides  these  are  well- 
developed  crystals  of  pale  yellowish  green,  strongly  pleochroic  pyroxene;  dark  brown 
hornblende  with  the  characteristic  black  border  in  not  so  sharply  outlined  forms; 
and,  as  accessory  minerals,  magnetite  and  apatite,  with  very  rarely  quartz,  mica, 
zircon,  and  tridymite. 

The  phenocrysts  of  feldspar,  including  all  that  do  not  take  part  in  the  ground- 
mass,  are  plagioclase.  The  largest  individuals,  reaching  4mm  in  length,  are  in 
crystals  nearly  equally  developed  in  the  direction  of  the  three  axes,  and  show  in  the 
sections,  besides  crystal  faces,  rounded  outlines.  They  are  not  abundant  in  the  rock 
sections  and  can  not  be  so  carefully  studied  optically  as  the  smaller  feldspars,  but 
from  those  that  are  met  with  it  appears  that  they  are  not  more  basic  than  labradorite, 
and  because  of  the  great  amount  of  glass  included  in  them  their  separation  and 
chemical  analysis  would  be  both  difficult  and  uncertain.  The  smaller  macroscopic 
individuals  have  well-defined  crystal  forms.  Their  sections  are  four,  five,  six,  and 
eight  sided  and  correspond  to  those  cut  from  crystals  with  OP,  ooPa,  «'P,  ooP', 
2  'Fob,  2  P'<5fc  faces.  They  are  for  the  most  part  prisms,  lengthened  in  the  direction  of 
the  brachydiagonal,  though  some  appear  tabular  in  the  plane  of  the  brachypinacoid. 
Irregularly  outlined  fragments  are  seldom  met  with.  A  very  marked,  sharply  defined 
zonal  structure  is  common  to  most  all  the  larger  crystals,  but  is  wanting  in  the  more 
minute  ones  of  the  groundmass.  The  cleavage  parallel  to  the  base  and  brachypinacoid 
is  not  very  generally  present  nor  very  perfect,  the  feldspar  having  the  irregular  frac- 
ture and  glassy  appearance  of  sanidine,  a  resemblance  still  more  striking  because  of 
the  nearly  total  absence  in  half  the  individuals  of  polysynthetic  twinning,  though  in 
almost  every  instance  an  apparently  simple  individual  or  Carlsbad  twin  is  found  to 
contain  one  or  more  thin  lamellae  of  feldspar  twinned  according  to  the  albite  law  or 
to  that  of  pericline.  The  medium  sized  individuals  seen  in  the  thin  sections,  which 
correspond  to  the  smallest  feldspars  noticed  in  the  hand  specimens,  from  0-5 mm  to 
l-0mm  in  length,  show  the  characteristic  polysynthetic  twinning  of  plagiodnse 
and  give  angles  of  extinction  symmetrical  to  the  composition  plane  as  follows: 
15°-15°,  30°-31°,  33°-33°,  33°-34°,  36°-39°,  which,  from  the  table  of  extinction-angles 
published  by  MM.  Fouque  et  Michel-Levy,1  correspond  to  those  of  anorthite  or  a 
feldspar  more  basic  than  labradorite.  The  smaller  individuals  are  twinned  after  the 
Carlsbad  law,  with  very  few  exceptions,  and  are  characterized  by  having  but  few 
lamella;,  of  short  length,  lying  in  two  directions  at  nearly  right  angles,  twinned  the 
one  after  albite  parallel  to  the  brachypinacoid,  the  other  after  pericline  parallel  to  the 
basal  cleavage  when  present.  lu  many  instances  the  lamella?  are  entirely  wanting, 
as  just  noticed.  A  careful  study  of  all  the  sections  that  showed  cleavage,  or  were 

'  Fouqu^  et  Michel-L6vy.     Mim-ralogie  Micrographique,  p.  228.     Paris,  1879. 


350  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

nearly  rectaugular  in  outline,  or  extinguished  symmetrically  with  respect  to  the  trace 
of  the  brachypinacoid,  gave  from  more  than  fifty  measurements  the  following  results 
in  sections  where  the  basal  cleavage  varied  not  more  than  5°  from  being  at  right 
angles  to  the  trace  of  the  brachypinacoid;  and  in  sections  without  cleavage,  almost 
rectangular,  the  angle  of  extinction  varied  from  30°  to  43°,  in  most  cases  being  about 
40°.  In  sections  with  symmetrical  extinction  it  was  25°,  34°,  36°,  38°,  40° — that  is, 
in  the  zone  perpendicular  to  the  brachypinacoid  the  angles  of  extinction  measured 
from  the  trace  of  the  latter  plane  reached  43°  and  were  mostly  greater  than  31°, 
showing  a  part  of  the  feldspar  to  be  anorthite. 

The  frequent  occurrence  in  this  andesite  of  nearly  rectangular  sections  of  twinned 
crystals  yielding  both  very  high  and  widely  varying  angles  of  extinction  led  to 
an  investigation  of  the  position  of  the  axes  of  elasticity  in  the  two  halves  of  sec- 
tions cut  from  Carlsbad  twins  of  plagioclase  in  a  zone  at  right  angles  to  the  brachy- 
pinacoid (aoP*).  From  the  nature  of  a  Carlsbad'  twin  it  is  evident  that  the  plane  of 
the  optic  axes  in  the  two  parts,  being  oblique  with  respect  to  the  vertical  crystallo 
graphic  axis  in  plagioclase  feldspars,  would  be  symmetrically  disposed  only  with 
respect  to  the  vertical  axis,  considered  as  its  axis  of  revolution;  hence  the  extinction 
angles  for  the  two  parts  of  the  twin,  that  is,  the  angles  on  a  cutting  plane  included 
between  the  trace  of  its  intersection  with  the  brachypinacoid  or  composition  plane 
and  the  traces  of  its  intersection  with  the  planes  of  the  optic  axes,  respectively, 
would  be  symmetrical  only  for  sections  in  the  zone  parallel  to  the  vertical  axis,  that  is 
in  the  zone  a>P<x,  ooPdo;  consequently  in  the  zone  at  right  angles  to  the  brachy- 
pinacoid there  will  be  only  one  position  where  the  extinction  angles  are  symmetrical, 
and  that  is  in  the  section  parallel  to  the  vertical  axis,  while  in  a  plane  perpendicular 
to  it  the  extinction  angles  will  be  complementary,  or  equal  when  measured  in  the  same 
direction:  that  is,  the  axes  of  elasticity  in  the  two  parts  will  be  respectively  parallel, 
but  in  all  other  sections  of  this  zone  they  will  be  unequal.  These  relations,  together 
with  the  degree  of  variation  in  the  extinction  angles  throughout  the  zone  may  be 
graphically  represented  by  the  following  diagram  (Fig.  8),  derived  from  the  curves  of 
extinction  angles  of  feldspars  in  the  zone  at  right  angles  to  the  brachypinacoid  pub- 
lished by  MM.  Fouque  et  Levy.1  The  case  of  labradorite  will  serve  as  an  illustra- 
tion. Fig.  7  represents  the  projection  of  a  Carlsbad  twin  of  that  species  on  the  plane 
of  the  brachypinacoid;  let  (a)  be  the  half  in  the  normal  crystallogaphic  position,  and 
(b)  that  in  the  twinned  position,  then  it  is  evident  that  in  considering  a  series  of  sec- 
tions perpendicular  to  the  plane  of  the  brachypinacoid,  if  we  pass  from  the  position  of 
a  normal  to  the  edge  OP,  <xP*  of  the  first  half  (a)  in  the  direction  of  the  obtuse  angle 
of  that  half,  we  at  the  same  time  pass  from  a  position  52°  from  the  normal  to  the  edge 
OP,  ccPdo  of  the  second  half  (b)  in  the  direction  of  the  acute  angle  of  that  half.  In 
Fig.  8  the  heavy  line  is  the  curve  of  the  extinction  angles  for  sections  of  the  first  half 

1  Min^ralogie  Micrographique,  etc. 


PYKOXENE-AXDES1TE. 


351 


(«) ;  commencing  at  the  normal,  X,  with  a  value  of  abont  30°,  the  direction  to  the  left 
corresponds  to  that  which  passes  over  the  obtuse  angle  of  that  half  (a);  the  light  line 
is  the  curve  for  the  second  half  (b),  its  corresponding  normal,  X',  being  52°  18'  to  the 
right  of  the  former,  and  starting  at  this  point  with  the  same  value,  30°,  but  with  oppo- 
site sign;  its  passage  to  the  left  corresponds  to  that  over  its  acute  angle.  From  the 


Flo.  7.— Carlsbad  twill  of  labradoritc. 


resulting  figure  it  is  readily  seen  that  in  the  plane  26°  9'  to  the  right  of  the  normal, 
X,  which  is  the  section  parallel  to  the  vertical  axis,  the  angles  of  extinction  in  the  two 
halves  are  equal  and  opposite,  that  is,  symmetrical  with  respect  to  the  brachypinacoid ; 
and  that  in  the  plane  63°  51'  to  the  left  of  the  same  normal,  X,  which  is  the  section 
at  right  angles  to  the  vertical  axis,  the  extinction  angles  are  equal,  but  have  the  same 


.X             n                                   -y            m                                 X              »                                  Y 

m 

00- 

z<£ 

^ 

-.-  * 

\ 

fc-l-* 

-=^r 

-^TTT 

~ 

—  -  - 

—  —  1 

mg 

— 

--,. 

^ 

--. 

fe 

-" 

10" 

•V 

., 

^ 

V 

/ 

\ 

. 

9.1 

B; 

-t 

i' 

f'l' 

- 

- 

10°  - 
20° 

/ 

/ 

'- 

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j 

,. 

,-' 

'N 

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4O'- 

i 

Y'                                                X' 

X*  Plane  0.1  right  angtcato  toP.  ofiPdb)  or<oJ               .X'JVan*  at,  "tght  angles  to  (oP.ooPdb)of(bt 
Y  Plan?  containing  foP.ooPtib)of  ta),                         ~Y'Plan,e  conta  ntng  /oPooPdbJo/"'W 

n  Plant;  parallel  to  vertical  axis 


m  Plane  at  right  angles  to  vertical  axt* 


Flo.  8.  —  Diagram  of  extinction  augleg. 

sign,  which  agrees  with  the  conclusions  previously  arrived  at.  The  greatest  differ- 
ence in  the  size  of  the  angles  in  any  one  section  appears  to  be  about  20°.  That  these 
variations  occur  in  a  great  number  of  nearly  rectangular  sections  is  understood  ujxui 
comparing  with  the  diagrams  the  following  table  of  angles  made  by  the  basal  cleavage 
and  the  trace  of  the  brachypinacoid  in  sections  in  the  zone  in  question.  The  figures 


352 


GEOLOGY  OF  THE  EUREKA  DISTRICT. 


in  the  second  column  denote  the  degrees  to  be  added  to  or  subtracted  from  90°  to  give 
the  required  angle  for  labradorite  in  sections  taken  every  5°  from  the  normal  to  the 
edge  OP,  ooPdb. 

Table  showing  the  angle  between  OP  cleavage  and  the  trace  of  ooP<X>  in 
planes  in  the  zone  perpendicular  to  ocP  ob  for  labradorite. 


Inclination  of 
plaue  to  that 
perpendicular 
to  the  edge 
OP,    »P<5b. 

Angle  to  be 
added  to 
or  subtracted 
from  90°. 

Angle  between  0  P  cleavage  and 
the  trace  of  ooP  &>. 

Obtuse  angle. 

Acute  angle. 

0° 

3°  20' 
3    20 
3    23 
3    26   40" 
3    32   20 
3    40 
3    50 
4      2    48 
4    20     6 
4    42   21 
5    13   20 
5    46   40 
6    36   40 
7    46   40 
9    36    40 
12    36   40 
18    28   42 
33    37 
48    27 
69    22 
90 

93°  20' 
94    42    21" 

180 

86°  40' 
85    17    39" 

0 

5 

10 

15 

20 

25 

30 

35 

40            

45          

50 

55 

60 

65 

70 

75 

80     

85  

87  

88 

90 

From  this  it  will  be  seen  that  the  variation  in  the  angles  made  by  the  cleavage 
is  only  1°  20'  for  45°  of  rotation  each  side  of  the  normal,  or  for  a  whole  quadrant,  but 
for  70°  on  both  sides  the  variation  is  only  about  6°.  Applying  this  to  Figures  7  and 
8  it  will  be  readily  seen  what  combinations  may  occur.  Thus  in  a  section  in  ttis 
zone  45°  to  the  left  of  X  the  cleavage  angle  will  be  94°  42'  in  the  half  (a)  and  about 
120°  in  the  half  (6),  and  whilst  the  extinction  angle  in  the  first  half  (a)  is  20°  that  in 
the  second  half  (b)  is  0°.  If  in  conjunction  with  this  Carlsbad  twinning  we  have  the 
polysynthetic  twinning  of  albite,  as  generally  happens,  we  shall  find  sections  in  the 
zone  under  discussion  one  side  of  which  will  show  striations  having  symmetrical 
extinction  angles  differing  from  the  symmetrical  extinction  angles  of  the  striations  in 
the  other  side  by  as  much  as  20°  in  some  cases,  a  phenomenon  which  might  lead  to 
the  erroneous  conclusion  that  two  species  of  plagioclase  feldspar  had  formed  together 
along  the  plane  of  the  brachypiuacoid.  It  is  possible  that  instances  of  such  an  occur- 
rence, which  have  been  mentioned  by  other  observers,  may  be  sections  of  Carlsbad 
twins  of  a  single  species. 

In  the  thin  sections  of  this  pyroxene-andesite  occur  many  examples  of  twinned 
feldspars,  in  nearly  rectangular  sections,  that  exhibit  optical  phenomena  similar  to 


PYKOXENE-ANUESITE. 


353 


\ 


those  just  described  for  labradorite,  but  which  differ  greatly  in  degree,  the  extinction 
angles  being  very  much  larger  than  those  of  labradorite  for  this  zone  as  given  by 
MM.  Fouque"  et  L6vy,  and  which  must  upon  this  ground  be  referred  to  anorthite.  An 
especially  fine  example  of  such  a  feldspar,  in  which  are  combined  the  three  sorts  of 
twinning  most  common  to  plagioclase — albite,  pericline,  and  Carlsbad — is  seen  in  thin 
section  79.  It  has  been  made  the  subject  of  a  series  of  careful  measurements,  which 

are  indicated  on  the  accompany- 
ing diagram  (Fig.  9).  It  con- 
sists of  two  nearly  equal  halves, 
twinned  after  the  Carlsbad  law, 
one  having  well-marked  cleav- 
age, which  is  absent  from  the 
other.  Each  shows  striations 
due  to  albite  twinning,  which 
give  symmetrical  extinction 
angles  that  are  n«»t  the  same 
for  the  two  halves.  Near  the 
middle  of  the  first  mentioned 
half  is  a  portion  twinned  after 
the  law  of  pericliue,  as  the 
cleavage  and  extinction  angles 
and  position  of  the  axes  of  elas- 
ticity show.  The  section  appears 
to  be  in  the  zone  perpendicular 
to  ooP&  and  nearly  parallel  to 
the  base  of  the  second  half. 
There  is  also  a  marked  zonal 
structure  and  variation  of  ex- 
tinction of  about  10°  from  the 
center  outward,  being  greatest 
at  the  center.  In  the  left-hand 
\lndicates^h&direcuon  of  the  pace  of  the  plane  half  the  symmetrical  extinction 
of.the  opcic  ax.es.  angles  in  the  marginal  zone 

^tnOtcatesthepoaittwi  of  the  interference  figure,  reach  30°  and  33°,  while  the 
F,O.  8-Carisbad  twin  of  piagiociase.  extinction  in  the  central  portim. 

is  40°  and  44°.  In  the  second  half  the  symmetrical  extinction  angles  are  11°  and  14° 
in  the  marginal  zone  and  24°  at  the  center.  This  variation  is  due  to  a  change  in  tin- 
position  of  the  axes  of  elasticity,  which  is  shown  by  the  fact  that  near  the  margin  of 
the  unstriated  end  of  the  first  half  the  hyperbolas  of  the  interference  figure  meet  in 
the  center  of  the  field,  but  near  the  center  of  the  same  portion  they  come  together  on 
the  edge  of  the  field. 
MON  xx 23 


354  GEOLOGY  OF  THE  EUBEKA  DISTRICT. 

The  phenomenon  of  zonal  variation  in  the  angle  of  extinction  of  feldspars 
indicates  that  the  chemical  composition  of  the  crystals  varies  from  the  center  out- 
wards. And  as  the  extinction  angle,  so  far  as  observed  in  the  feldspars  of  the  ande- 
site  of  this  district,  is  usually  greater  at  the  center  of  the  crystal  than  toward  the 
margin,  generally  passing  through  a  series  of  distinctly  marked  zones,  which  in  rare 
instances  have  been  found  to  differ  by  20°,  yet  passing  frequently  by  imperceptible 
gradations  from  one  extreme  to  the  other,  it  seems  likely  that  during  the  growth  of 
such  feldspars  changes  have  occurred  in  the  chemical  composition  of  the  successive 
shells  of  enlargement,  tending  toward  greater  acidity,  which,  though  often  sharply 
denned  or  interrupted,  have  sometimes  taken  place  in  the  most  gradual  manner  possi- 
ble, a  process  only  conceivable  by  admitting  the  correctness  of  Tscherinak's  theory. 
The  particular  section  of  twinned  feldspar  described  and  illustrated  in  Fig.  3  has  been 
treated  with  hot  hydrochloric  acid.  The  central  portion  of  both  halves  was  decom- 
posed and  clouded  and  the  zonal  structure  more  strongly  emphasized.  The  marginal 
zones  appeared  to  resist  the  attack  of  the  acid  completely.  This  proves  that  the  cen- 
tral portion  of  the  first  half,  with  extinction  angles  as  high  as  40°  and  44°,  is  anorthite 
or  bytownite,  and  that  the  central  portion  of  the  second  half  is  of  the  same  species, 
but  was  cut  in  a  position  in  which  the  extinction  was  only  24°.  The  outer  zones  are 
probably  labradorite.  The  difference  of  their  behavior  toward  hydrochloric  acid  is 
more  striking  than  their  optical  difference. 

The  occurrence  of  auorthite  in  the  volcanic  rocks  of  western  America  has  not 
been  previously  noticed,  partly  because  no  very  thorough  investigation  of  the  nature 
of  the  plagioclase  feldspar  in  them  has  been  undertaken  und  also  from  the  fact  that 
all  simple  crystals  showing  no  stria?  between  crossed  nicols,  were  classed  with  ortho- 
tomic  feldspar.  Thus  the  simple  crystals  and  Carlsbad  twins  of  sauidine  mentioned  in 
Prof.  Zirkel's  report  on  the  rocks  of  the  40th  Parallel  Survey,1  as  occurring  in  such 
abundance  in  the  "  augite-andesite"  at  Basalt  Creek,  Washoe,  and  near  Chirks  Station 
and  Wadsworth,  near  the  Truckee  River,  give  in  the  zone  perpendicular  to  the  brachy- 
pinacoid  angles  of  extinction  ranging  from  0°  in  a  few  instances  to  40°,  thus  3!?°,  34°, 
35°,  36°,  38°,  39°,  40°,  most  of  the  reading  being  over  30°,  corresponding  to  those  of 
anorthite.  One  section  cut  at  right  angles  to  'an  optic  axis  showed  tin-  plane  of  the 
optic  axes  at  an  inclination  of  43°  to  the  trace  of  the  brachypinacoid.  Similar  auor- 
thite is  found  in  the  -closely  related  andesites  in  the  Cortez  Range,  head  of  Annies 
Creek,  and  on  Emigrant  Road,  Palisade  Canyon,  and  also  from  the  Traverse  Mountain, 
Utah.  It  occurs  in  the  "  augite  trachyte,  "*  from  the  neighboring  Wall weah  Range, 
in  the  "  trachytes" '  from  Emigrant  Road  and  the  south  bank  of  Palisade  Canyon, 
Cortez  Range,  and  in  the  rock  from  Jacobs  Promontory,  Shoshone  Range,  erroneously 
determined  as  rhyolite,1  which  is  almost  identical  with  the  andesite  from  Richmond 
Mountain.  It  will  thus  be  seen  that  auorthite  has  a  very  wide  geographical  distri- 


'F.  Zirkel:  Micro.  Petro.,  U.  S.  Expl.  40th  Par.,  vol.  vi,  Washington,  1876. 


PYKOXENE-ANDESITE.  355 

bution  in  the  West,  though  the  rocks  containing  it  can  be  shown  to  be  of  the  same 
character  throughout. 

The  largest  individuals  are  characterized  by  a  great  abundance  of  glass  inclu- 
sions, which  extend  from  the  center  outward,  always  leaving  a  border  of  feld- 
spar free  from  inclusions.  They  are  very  irregular  in  outline  and  form  a  net-work  so 
thick  in  many  instances  as  to  equal  in  amount  the  feldspar  which  forms  the  meshes. 
The  glass  is  colorless  and  filled  with  opaque  grains  and  transparent  globulites,  besides 
colorless  microlites,  whose  high  index  of  refraction  and  similarity  to  other  more 
determinate  ones  in  the  groundmass  suggest  their  pyroxenic  nature.  There  also 
occur  inclusions  of  the  groundmass  developed  to  the  same  degree  as  that  surrounding 
the  feldspar  crystal.  The  smaller  individuals  are  freer  from  inclusions,  but  contain  a 
greater  variety,  the  glass  ones  having  sharp  outlines,  either  round  or  nearly  rectan- 
gular, with  a  comparatively  large  gas-bubble  and  fewer  microlitic  secretions;  liquid 
inclusions  are  less  frequent,  with  a  briskly  moving  bubble,  besides  needles  and  stouter 
prisms  of  apatite,  magnetite  grains  and  rarely  augite.  The  feldspar  substance  is 
entirely  fresh,  without  the  slightest  trace  of  decomposition;  in  some  instances  it  is 
intersected  by  cracks,  in  which  hydrous  oxide  of  iron  has  been  deposited,  and  which 
have  led  to  the  devitrification  of  part  of  the  included  glass,  converting  it  into  a  yellow 
cryptocrystalline  aggregate.  One  single  individual  contained  calcite  deposited  along 
Hues  of  fracture.  There  is  also  present  among  the  phenocrysts  feldspars  with  quite 
perfect  cleavage,  splinters  of  which  parallel  to  the  base  give  an  angle  of  extinction  of 
0°  and  are  probably  oligoclase,  their  separation  from  anorthite  by  optical  methods  is 
not  possible  in  the  thin  section. 

The  microscopic  lath-shaped  feldspar  crystals  of  the  groundmass,  averaging 
0-03lnm  in  length  by  O-OOS0""  in  breadth  are  slender  prisms  elongated  in  the  direction 
of  the  brachydiagonal,  irregularly  terminating  in  two  or  more  needles  of  different 
lengths  and  are  in  every  case  twinned  with  two  or  three  lamelhe.  The  angle  of 
extinction  measured  from  the  direction  of  their  length  varies  from  0°  to  26°  and  cor- 
responds to  labradorite  or  a  less  basic  feldspar.  Small  square  sections,  not  very 
abundant,  prove  by  their  diagonal  extinction  to  belong  to  plagioclase. 

The  second  most  essential  component  is  pyroxene,  which  occurs  in  macroscopic 
crystals  averaging  lmm  in  length,  a  few  reaching  2mm  from  which  they  diminish  in 
size  to  -03inm,  having  sharply  defined  outlines,  well  developed  faces  in  the  prism  zone, 
of  which  the  pinacoidal  are  much  the  larger,  and  occasionally  showing  the  pyramid 
P  and  rarely  the  base  OP.  The  larger  number  of  individuals,  however,  are  not 
crystallographically  outlined,  but  appear  as  imperfectly  developed  crystals  in  more  or 
less  rounded  forms.  It  is  without  the  black  border  that  surrounds  the  hornblende, 
but  has  a  narrow  granular  margin  of  pale  yellow  transparent  grains,  without  doubt 
augite  of  final  crystallization,  formed  at  the  time  of  solidification  of  the  gmundinass 
about  the  primary  larger  individuals  and  to  a  lesser  degree  around  the  black  bordered 
hornblendes  and  magnetite  grains,  but  in  no  instance  altont  the  feldspars.  Its 


356 


GEOLOGY  OF  THE  EUltEKA  DISTKICT. 


presence  is  not  universal,  some  pyroxenes  being  entirely  free  from.  it.  In  a  very  few 
instances  an  uncompleted  black  border  has  been  added  to  the  primary  augite,  in  every 
case  projecting  beyond  the  crystal  outline  of  tlie  remainder  of  the  surface,  Fig.  3,  PL 
in,  and  being  inclosed  in  the  narrow  margin  just  described.  This  black  border  appears 
to  be  an  aggregation  of  magnetite  grains.  A  zonal  structure  is  occasionally  noticed. 
The  prismatic  cleavage  parallel  to  ocP  is  quite  perfect  in  some  crystals,  but  in  others 
it  is  nearly  lost  in  irregular  fractures.  The  crystals  are  mostly  simple  individuals;  a 
few  are  twinned  parallel  to  the  orthopiuacoid  and  show  three  or  four  alternating  bands 
between  crossed  nicols. 

At  the  time  when  these  rock  sections  were  studied  it  was  considered  probable 
that  all  the  pyroxene  individuals  observed  in  any  one  rock  belonged  to  the  same 
species,  and  that  those  sections  with  the  axes  of  elasticity  parallel  to  their  cleav- 
age or  to  the  trace  of  the  faces  in  the  prism  zone  were  sections  cut  iu  the  zone  at  right 
angles  to  the  cliiiopinacoid  of  augite,  when  they  were  accompanied  by  other  sections 
with  inclined  position  for  these  axes.  Hence  all  the  pyroxene  in  this  case  was  thought 
to  be  augite.  But  the  observations  of  Cross1  on  the  hypersthene-andesites  of  Colo- 
rado and  oth.er  localities,  and  our  own  observations  on  the  andesites  of  the  volcanoes 
of  northern  California,  Oregon,  and  Washington  Territory,2  and  on  the  volcanic  rocks 
of  the  Great  Basin,3  and  the  studies  of  many  other  observers,  in  different  parts  of  the 
world  have  demonstrated  the  joint  occurrence  of  an  orthorhombic  and  a  monoclinic 
pyroxene  in  a  great  variety  of  rocks.  Moreover,  the  pyroxene  of  this  particular  ande- 
site  from  Kichmond  Mountain  has  been  separated  from  the  rock  by  means  of  the 
cadmiumborotuagstate  solution,  as  already  described  iu  the  paper  on  the  volcanic 
rocks  of  the  Great  Basin  just  mentioned.  The  pyroxene  was  found  to  consist  of  green 
augite  and  brown  hypersthene;  the  latter  was  isolated  with  a  small  admixture  of  the 
augite  and  analyzed.  From  the  composition  of  the  whole,  analysis  I,  a  theoretical 
composition  for  the  hypersthene  and  augite  was  calculated,  resulting  as  follows : 


I. 

Mixture. 

II. 
Hypers- 
thene. 

III. 

Augite. 

SiO2  

51-16 

51-39 

49-02 

A12O3  

3-50 

3-26 

5-64 

TiOj  

•73 

•73 

•73 

FeO  

15-46 

16-45 

6-45 

MnO  

•56 

•56 

•56 

MgO  

19-22 

19-75 

14-37 

CaO  

8-84 

7-31 

22-60 

IKH  .  . 

•42 

•42 

•42 

99-89 

99-87 

99-79 

1  Am.  Jour.  Sci.,  1883,  vol.  xxv,  pp.  139-144. 

2  Am.  Jour.  Sci.,  Sept.,  1883,  vol.  xxvi. 

3  Am.  Jour.  Sci.,  June,  1884,  vol.  XXVH, 


PYROXENE-ANDESITE.  357 

The  percentage  of  FeO  being  greater  than  14  per  cent  the  orthorhombic  pyroxene 
may  be  classed  as  hypersthene.  The  optical  character  was  determined  in  the  isolated 
crystals  and  corresponded  to  hypersthene. 

A  review  of  the  thin  sections  of  the  andesite  from  Richmond  Mountain  shows  that 
the  two  pyroxenes  resemble  one  another  closely  in  thin  sectioir,  but  the  hypersthene 
is  pleochroic  to  a  greater  or  less  extent,  the  augite  not  at  all  so.  The  pleochroism  of 
the  hypersthene  is,  of  course,  stronger  in  the  thicker  sections,  but  varies  among  the 
individuals  in  a  single  section  and  in  some  instances  differs  zonally  in  a  single  crystal, 
being  stronger  in  the  central  portion  of  some  individuals  and  in  the  marginal  portions 
of  others.  It  is  green  parallel  to  the  c  axis  and  light  brown  parallel  to  a  and  b  with  a>b. 
In  some  cases  they  are  nearly  colorless.  The  augites  are  very  light  yellowish  green  to 
colorless.  Cleavage  parallel  to  the  prism  and  more  rarely  to  the  pinacoids  is  observed 
in  cross  sections  cut  perpendicular  to  the  positive  bisectrix;  but  in  many  longitudinal 
sections  there  is  no  trace  of  cleavage. 

The  slight  border  of  augite  grains  surrounding  many  of  the  pyroxenes  is  almost 
exclusively  confined  to  the  porphyritic  augite  crystals.  This  is  most  noticeable  where 
both  varieties  of  pyroxene  have  grown  together  in  parallel  crystallographic  orienta- 
tion, the  hypersthene  being  the  older  secretion  in  most  every  case;  the  granular 
augite  border  extends  around  the  augite  crystal,  but  ceases  at  the  hypersthene.  The 
orthorhombic  pyroxene  is  more  readily  altered  than  the  augite,  a  fibration  parallel  to 
the  c  axis  sets  in  from  the  surface  and  along  the  cracks,  resulting  in  a  light  green, 
highly  refracting  mineral  with  an  inclined  extinction  angle  which  reaches  15°,  and  is 
evidently  a  fibrous  hornblende  (actinolite).  The  crystals  are  sometimes  coated  with 
brown  oxide  of  iron  (limonite),  which  also  coats  the  pyroxene  micro!  ites  and  porphy- 
ritical  hornblendes.  Though  generally  free  from  inclusions  some  individuals  bear 
numerous  magnetite  grains,  and  irregularly  shaped,  colorless  glass  inclusions  with  a 
gas  bubble,  besides  apatite  needles  and,  rarely,  imperfectly  formed  brown  hornblende. 

The  pyroxene  microlites  of  the  groundmass,  varying  from  0-04  or  0-05 mm  in 
length  to  microscopically  minute  proportions,  are  long  slender  prisms  parallel  to  the 
vertical  axis,  terminated  by  a  pyramid.  They  are  of  a  pale  greenish  color  and  con- 
tain numerous  magnetite  grains,  which  are  in  no  case  associated  with  the  feldspar 
microlites.  Their  augitic  nature  is  shown  by  their  crystalline  form,  color,  and  high 
index  of  refraction,  taken  in  connection  with  their  angle  of  extinction,  which  varies 
from  0°  to  more  than  35°,  being  indeed  directly  traceable,  through  occasional  larger 
individuals,  to  those  of  unquestionable  augitic  nature.  A  part,  however,  may  be 
hypersthene.  The  parallel,  fibrous  decomposition  product  is  in  one  instance,  No.  90, 
colored  red  by  oxide  of  iron,  producing  small  prisms  of  a  reddish  yellow  color,  pre- 
cisely similar  to  those  mentioned  by  Prof.  Zirkel  as  of  an  indeterminable  nature  in 
the  "trachyte"  from  the  south  bank  of  Palisade  Canyon,  Cortez  Range,  previously 
referred  to,  which  are  there  also  traceable  to  augite.  This  microscopic  angite  <>t'  tinul 


358  GEOLOGY  OF  THE  EITKEKA  DISTRICT. 

crystallization  appears  more  readily  altered  than  the  macroscopic  primary  crystals, 
and  when  discolored  by  iron  oxide  forms  dark  red,  narrow  borders  around  the  still 
fresh  larger  augites  and  black  bordered  hornblendes,  suggesting  the  characteristic 
black  border  of  the  latter  mineral,  from  which,  however,  it  is  easily  distinguished. 
Aggregations  of  augitfe  crystals  around  a  foreign  nucleus  are  occasionally  met  with. 

The  hornblende  of  this  rock  is  quite  abundant  in  crystals,  which  are  not  very 
well  developed,  except  in  the  prism  zone,  where,  besides  the  ordinary  faces,  ooP  and 
ooP  &,  there  is  occasionally  the  orthopinacoid,  ooP  db.  The  terminal  faces  are  not 
recognizable  iu  the  thin  sections  studied,  but,  judging  from  the  macroscopic  crystals 
in  the  hand  specimens,  they  appear  to  be  those  usually  developed.  The  majority  of 
individuals  seen  under  the  microscope  are  irregularly  outlined.  The  largest  reach 
4  to  5"""  in  length,  but  the  greater  number  average  less  than  lmm.  They  do  not  take 
part  in  the  composition  of  the  grouudmass.  The  cleavage  is  very  perfect,  parallel  to 
the  prism,  forming  a  very  sharp  network  of  parallel  lines,  thus  differing  from  the 
pyroxene,  in  which  the  less  perfect  cleavage  is  combined  with  irregular  cracks.  There 
is  in  some  instances  a  second  cleavage,  parallel  to  the  clinopinacoid,  never  well  devel- 
oped. Some  of  the  individuals  are  twinned  in  the  usual  manner  parallel  to  the 
orthopinacoid.  The  hornblende  is  dark  reddish  brown  in  color,  with  a  strong  absorp- 
tion. In  the  dark  and  more  resinous  varieties  of  the  audesite  (Nos.  77,  78,  79)  the 
color  is  reddish  brown,  being  dark  brown  parallel  to  the  axis  of  least  elasticity  (c), 
nearly  the  same  shade  of  brown  parallel  to  the  axis  of  mean  elasticity  (b),  and  light 
yellowish  brown  parallel  to  the  axis  of  greatest  elasticity  (a);  that  is,  c  =  dark  brown, 
b  =  dark  brown,  a  =  yellowish  brown,  and  c  =  b>a,  possibly  c  >  b  >  a.  In  the  lighter 
colored,  purple  and  fissile  varieties  of  the  audesite  (Nos.  85,  86)  the  pleochroism  is 
greater,  but  the  absorption  less,  the  brown  color  having  a  greenish  tinge  and  the 
pleochroism  being  as  follows:  Parallel  to  c  browish  green,  parallel  to  b  reddish  brown, 
parallel  to  a  yellow,  and  c  =  b>a.  In  the  specimen  from  Trail  Hill  (No.  90)  the  color 
parallel  to  c  is  brownish  red,  parallel  to  b  brown,  parallel  to  a  light  brown,  c  >  b  >  a. 

The  hornblende  individuals  are  surrounded  by  an  opaque  black  border  that 
bounds  the  whole  outline  of  each  section,  the  fractured  or  eroded  portions  in  the  same 
manner  as  the  crystal  faces ;  its  width  varies  somewhat,  and  is  not  constant  for  any 
one  individual.  It  is  quite  sharply  denned,  both  on  the  outside  and  inside,  though 
occasionally  it  is  seen  shading  into  the  hornblende  substance  as  minute  opaque  dust. 
It  appears  to  be  magnetite,  having  the  same  luster  in  incident  light  and  the  same 
products  of  decomposition,  hydrous  oxide  of  iron.  Spots  of  similar  magnetite  dust 
occur  inclosed  in  the  hornblende,  besides  the  inclusions  of  coarser  grains  and  crystals 
of  magnetite,  sometimes  arranged  in  lines  parallel  to  the  clinopinacoidal  cleavage. 
The  fact  that  the  black  border  does  not  occur  between  the  hornblende  and  feldspar 
or  augite  when  they  are  in  contact,  but  always  between  hornblende  and  the  ground- 
mass,  together  with  the  fact  that  it  surrounds  the  fractured  portions  and  lines  the 


PYROXENE-ANDESITE.  359 

intruding  bays  of  grounduiass  in  crystals,  with  more  or  less  rounded  angles,  and  that 
the  outline  of  the  border  is  generally  that  of  the  crystal,  while  that  of  the  hornblende. 
substance  within  is  mostly  irregular,  suggests  its  being  the  result  of  a  change  in  the 
condition  of  the  molten  magma  when  hornblende  ceased  to  crystallize  out  and  pre- 
viously formed  .hornblende  crystals  "may  have  been  partially  melted,  or  replaced  by 
magnetite.  This  has  m  some  instances  proceeded  so  far  as  to  form  pseudomorphs  of 
magnetite  after  hornblende  (thin  sections  90, 91, 87,  88),  as  noticed  by  other  observers. 
Some  oi  the  pseudomorphs  (thin  section  91)  show  minute  grains  of  augite  uniformly 
mingled  with  the  magnetite,1  suggesting  more  strongly  that  there  has  been  a  melting 
of  the  hornblende,  followed  by  recrystallizatiou,  under  conditions  which  led  to  the 
production  of  augite  in  place  of  the  hornblende.  This  corresponds  to  the  results 
obtained  in  the  artificial  reproduction  of  hornblende,  in  which  augite  has  always 
been  formed  instead  of  hornblende.  The  hornblende  is  very  free  from  inclusions,  for 
besides  magnetite,  only  a  small  amount  of  colorless  apatite  is  found,  and  in  one  or 
two  cases  feldspar  and  augite.  It  is  absolutely  fresh  in  all  the  sections  made  from 
Richmond  Mountain ;  as  remarked  before,  it  is  not  a  constituent  of  the  grounduiass. 

Magnetite  is  less  abundant  than  the  minerals  just  described,  and  of  much  less 
importance  in  the  composition  of  the  rock,  yet  at  the  same  time  it  is  a  constant  ingre- 
dient. It  occurs  in  crystals  and  irregularly  shaped  grains,  the  largest  about  0-2 mm  in 
size,  from  which  they  range  to  almost  indistinguishable  grains  in  the  groundmass; 
it  is  very  evenly  disseminated,  but  not  very  .abundant. 

Apatite  is  another  constant  factor,  though  of  little  importance;  it  occurs  in  com- 
paratively large,  stout  crystals,  0-2 Inm  long  by  0-05 mm  broad,  giving  sharp  hexagonal 
cross  sections  and  showing  in  longitudinal  sections  the  pyramidal  termination,  P.  It 
is  colorless,  but  in  some  instances  is  crowded  with  opaque  microlites  arranged  parallel 
to  the  vertical  axis  of  the  crystal.  These  give  it  a  brown  or  gray  dusted  appearance 
and  exhibit  an  absorption  parallel  to  the  longest  axis.  One  cross  section  shows  these 
microlites  arranged  parallel  to  the  longest  axis  and  in  planes  parallel  to  the  prism 
faces  (section  79).  The  apatites  also  contain  a  few  inclusions  of  glass  with  gas  bubbles, 
which  are  in  negative  crystal  cavities.  The  apatite  is  found  closely  associated  with 
the  pheuocrysts  and  seldom  alone  in  the  groundmass. 

As  accessory  minerals  biotite  ranks  first  in  importance,  being  of  special  interest 
on  account  of  its  scarcity  in  this  pyroxeue-andesit<  of  Richmond  Mountain  and  in  the 
similar  pyroxene-andesite  of  Cliff  Hills,  as  compared  with  its  great  abundance  in  the 
hornblende-rnica-andesite  and  andesitic  pearlite  of  the  district.  It  is  found  in  only  two 
thin  sections  from  Richmond  Mountain  (Nos.  79,  78),  and  in  each  of  these  there  is  only 
a  single  individual  of  rounded  form  with  intruding  bays  of  groundmass.  The  mineral 
is  brown,  with  strong  absorption,  and  is  filled  with  minute  grains  of  magnetite  deposited 

'The  sarno  .ilw.-rvation  has  been  made  by  Dr.  K.  Oebbeke:  Beitrftge  »nr  Petrographie  der  Phillpplnen  and  dor 
Palau-Insel.    Neues  Jahrbuch  far  Min.,  etc.,  1881.    B.  B.  I,  p.  474. 


360  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

along  the  lines  of  cleavage.  An  exceptional  occurrence  of  mica  is  found  in  the  andesite 
exposed  southeast  of  Trail  Hill  (No.  91).  It  does  not  form  macroscopic  crystals,  but 
occurs  in  small,  irregular  patches,  closely  associated  with  the  macroscopic  augite,  and 
also  in  more  or  less  regular  plates,  quite  uniformly  disseminated  through  the  groundmass. 
It  is  brown  and  has  a  strong  absorption,  showing  a  large  angle  between  the  optic 
axis,  and  appears  in  so  fresh  a  rock  to  be  undoubtedly  of  primary  origin.  A  similar 
occurrence  is  noted  in  the  exceptional  ''augite-andesite"  from  Palisade  Canyon,  Cortez 
Range,  described  by  Prof.  Zirkel.1  The  two  rocks,  however,  appear  quite  different 
both  in  the  hand  specimen  and  under  the  microscope.  The  latter  is  coarsely  crystal- 
line and  contains  plagioclase,  quartz,  hypersthene,  and  brown  mica,  while  the  former 
has  a  microcrystalline  groundmass  with  porphyritical  crystals. 

Quartz  phenocrysts  are  very  rare.  Two  rounded  grains  of  very  pure  quartz 
without  inclusions  are  found  in  thin  section  87.  An  irregular  grain  containing  some 
fluid  inclusions,  with  briskly  moving  bubbles,  in  thin  section  86,  exhibits  a  varying 
optical  orientation,  plainly  arising  from  unequal  tension  throughout  the  individual. 
It  is  found  in  the  groundmass  of  the  holocrystalline  varieties  (91-97),  as  the  last  min- 
eral to  crystallize,  forming  a  cement  for  the  other  constituents.  It  can  be  determined 
optically  as  a  positive  uniaxial  mineral.  It  contains  numerous  glass  and  gas  inclu- 
sions. Its  outline  is  very  irregular,  as  the  quartz  individual  extends  among  the 
neighboring  feldspar  grains  for  some  little  distance,  producing  an  irregular  patch  of 
quartz  substance,  which  becomes  alternately  dark  and  light  throughout  its  whole 
extent,  as  the  thin  section  is  rotated  between  crossed  iiicols — a  micropoikilitic  structure. 

Tridymite  is  very  abundant  in  the  vesicular  forms  of  this  andesite,  thin  sections 
90,  87,  88.  It  occurs  as  microscopic  aggregates  of  hexagonal  plates  about  0-02 mm  in 
diameter,  filling  small  amygdaloidal  cavities  and  incrusting  the  walls  of  larger  ones 
with  easily  recognizable  macroscopic  crystals.  Tridymite  has  been  found  by  Prof. 
Zirkel  in  the  precisely  similar  rock  froin  the  south  bank  of  Palisade  Canyon,  Cortez 
Range,2  and  in  the  rock  from  the  same  locality,3  before  noticed  in  connection  with  the 
occurrence  of  anorthite. 

The  groundmass  of  these  andesites  has  the  "felt-like"  structure  noticed  by 
Prof.  Zirkel  as  characteristic  of  "  augite-andesite."  It  consists  of  a  colorless  glass 
base  crowded  with  microlites  of  feldspar  and  augite,  with  minuter  crystals  of  magi.et- 
ite  associated  with  the  augite,  besides  more  or  less  dark  colored  globulites  of  an 
indeterminable  nature,  the  whole  generally  showing  a  marked  flow-structure.  The 
proportion  of  glass  base  to  microlites  varies  in  different  localities  on  Richmond 
Mountain.  It  is  most  abundant  in  the  dark  resinous  variety  (Nos.  77,  78,  79), 
where  it  is  nearly  equal  to  the  microlites  in  amount.  The  gray  color  in  these 
thin  sections  appears  to  be  due  to  minute  magnetite  grains,  together  with  augite 

T.  Zirkel.    Micro.  Petro..  U.  S.  Kxpl.  40th  Par.,  vol.  vi,  p.  227,  No.  527. 
*Op.  cit.  specimen  No.  311. 
>Op.  cit.  specimen  No.  310. 


PYROXENE  ANDESITE.  $61 

microlites,  the  reddish  tint  of  the  other  varieties  (Nos.  85,  86,  87,  88,  90,  91) 
arising  from  the  presence  of  a  higher  oxide  of  iron  incrusting  the  magnetite.  In  the 
first  mentioned  variety  the  number  of  augite  microlites  exceeds  that  of  the  feldspar. 
In  the  lighter  colored  fissile  forms  (Nos.  85,  86)  the  feldspar  is  in  excess  and  the  glass 
base  is  not  so  abundant.  In  the  vesicular  andesite  the  composition  of  the  groundmass 
is  not  homogeneous  throughout,  for  besides  the  amygdules  of  tridymite  are  light 
colored  spots  where  the  augite,  magnetite  and  globulites  are  almost  wholly  wanting 
(No.  88).  Glass  base  is  altogether  absent  from  the  mica-bearing  groundmass  of  thin 
section  91,  which  is  nricrocrystalline,  with  grains  and  lath-shaped  microlites  of  feldspar 
cemented  together  with  quartz.  An  exceptional  red  variety  is  found  in  which  the 
colorless  glass  base  is  so  thickly  crowded  with  red  oxide  of  iron  as  only  to  be  detected 
in  the  thinnest  possible  section  (No.  92). 

(&.)  The  pyroxene-andesite  of  Cliff  Hills  is  identical  with  that  of  Richmond 
Mountain;  it  shows  the  same  modifications  in  the  field  as  the  latter,  corresponding  to 
which  are  the  same  microscopic  characters.  Thin  section  102  is  from  a  resinous  blue- 
black  variety  similar  to  Nos.  77,  78,  79  of  Richmond ;  section  107  is  from  a  reddish 
purple  form,  and  corresponds  to  No.  90  from  Trail  Hill.  Thin  section  108  is  like  No. 
92,  and  the  remaining  two  sections,  104  and  109,  are  slightly  modified  varieties.  Under 
the  microscope  the  typical  andesite  has  a  gray  groundmass  of  glass  with  microlites  of 
feldspar  and  augite  and  an  abundance  of  magnetite.  It  bears  phenocrysts  of  feldspar, 
augite,  hypersthene,  and  black  bordered  hornblende. 

The  feldspar  is  triclinic  without  any  admixture  of  recognizable  orthoclase,  the 
individuals  are  all  striated  by  multiple  twinning.  Their  outline  is  mostly  rectangular, 
some  with  the  angles  truncated  or  rounded,  indicating  their  form  to  have  been  prisms  in 
the  direction  of  the  brachydiagonal,  having  the  faces  OP,  oo  Pdb,  coP',  GO 'P,  2 PS.  The 
largest  phenocrysts  are  developed  more  equally  in  the  direction  of  the  three  axes;  the 
feldspar  microlites  in  the  groundmass  are  wholly  lath  shaped.  The  angles  of  extinction 
of  the  porphyritical  crystals  reach  35°,  40°,  and  44°  in  the  zone  at  right  angles  to  the 
brachypinacoid,  which  correspond  to  anorthite,  as  does  also  the  high  light  they  exhibit 
between  crossed  nicols  in  very  thin  sections.  Optically  it  can  not  be  determined 
whether  other  species  of  triclinic  feldspar  are  at  the  same  time  present  among  the 
larger  phenocrysts,  unless  the  great  divergence  of  extinction  angles  in  the  zonally 
built  individuals,  which  reaches  in  one  instance  32°  (102),  be  taken  as  evidence  of 
difference  in  chemical  composition  between  the  different  zones.  The  zonal  struc- 
ture is  beautifully  developed  in  some  individuals,  especially  so  in  the  crystal  just 
referred  to,  and  also  in  another  in  the  same  thin  section,  Fig.  C,  PI.  III.  Where  the 
inner  zone  has  a  sharp  crystallographic  outline,  while  the  outer  one  is  rounded  at 
the  corners,  the  angle  of  extinction  for  the  former  being  38°  and  for  the  latter  only 
18°,  narrow  strips  of  twinned  feldspar  pass  through  the  different  zones,  without  tak 
ing  part  in  the  zonal  structure!,  and  having  the  same  angle  of  extinction  throughout 


362  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

The  individuals  show  both  the  polysynthetic  twinning  of  albite  and  of  pericline, 
besides  the  simple  Carlsbad  twinning,  which  is  often  shown  by  the  outline  of  the 
sections,  but  the  striae  are  in  many  cases  few  in  number,  and  are  sometimes  altogether 
wanting. 

The  larger  feldspar  crystals  are  especially  rich  in  inclusions,  which  are  massed 
in  the  center  or  arranges  in  concentric  zones,  or  are  scattered  irregularly  through  the 
crystal.  A  good  example  of  the  zonal  arrangement  is  seen  in  thin  section  107.  The 
zone  of  inclusions  in  every  case  consists  of  minute  particles  of  glass  carrying  globu- 
lites  and  possibly  gas  bubbles,  so  densely  crowded  as  to  exceed  in  amount  the  inclos- 
ing feldspar  substance;  when  occurring  scattered  their  form  is  seen  to  be  in  some 
cases  very  irregular;  in  others  rectangular,  with  the  edges  parallel  to  the  outlines  of 
the  feldspar  crystal.  In  thin  section  102,  there  are  brown  and  gray  globulitic  glass 
inclusions  bearing  augite  inicrolites,  besides  which  are  isolated  colorless  glass  inclu- 
sions with  gas  bubbles,  and  an  occasional  microlite.  There  are  also  iuclosures  of  the 
groundmass  and  of  the  associated  inicrolites.  The  smaller  crystals  are  much  freer 
from  inclusions.  The  lath-shaped  feldspar  microlites  forming  the  groundmass  are 
unevenly  terminated  and  twinned  in  two  or  three  stripes;  the  angle  of  extinction  is 
in  general  low,  sometimes  reaching  the  limit  of  labradorite,  to  which  species  they  seem 
to  belong  in  part,  though  it  is  probable  that  a  less  basic  species  is  also  present. 

Pyroxene  is  abundant  both  as  macroscopic  crystals  and  as  microlites  in  the 
groundmass,  its  crystals  are  prisms,  frequently  very  long  and  slender,  with  the  prism 
zone  well  developed ;  the  pinacoidal  faces  are  much  larger  than  the  prismatic ;  the  cleav- 
age is  poor,  and  there  are  many  irregular  fractures.  The  twinning  is  that  ordinarily 
met  with.  The  pleochroism  of  the  hypersthene  is  strong,  but  varies  greatly  among 
the  individuals  in  one  and  the  same  rock  section,  in  some  cases  being  scarcely  per 
ceptible.  The  absorption  and  pleochroism  are  green  parallel  to  c,  light  reddish  brown 
parallel  to  a.  In  sections  at  right  angles  to  the  vertical  axis  the  colors  are,  yellow 
parallel  to  a  and  grayish  purple  parallel  to  b,  that  is  c=green,  o=light  reddish  brown 
to  yellow,  b= grayish  purple.  Sections  apparently  in  the  same  cry stallographic posi- 
tion vary  greatly  in  their  degree  of  coloring.  They  are  poor  in  inclusions,  of  which 
the  most  characteristic  are  magnetite  grains,  apatite  needles  and  glass.  There  is  around 
most  of  the  augite  crystals  a  narrow  border  of  augite  grains  of  final  crystallization, 
which  also  surrounds  the  black  border  of  hornblende  and  magnetite  as  previously 
described;  some  individuals  are  entirely  free  from  it,  and  a  very  few  have  a  partial 
black  border  like  hornblende,  Fig.  2,  PI.  in.  It  is  especially  noticeable  in  thin 
section  104,  where  of  two  pyroxene  crystals  almost  in  contact  one,  an  augite,  has  a  com- 
plete border  of  magnetite,  partially  altered  to  red  oxide,  while  the  other,  a  hyper- 
sthene, has  no  border  whatever.  The  decomposition  of  the  pyroxene  results  iu  the 
same  yellow  fibrous  mineral  mentioned  under  the  Richmond  Mountain  andesite.  The 
granular  augite  border  and  the  smaller  augite  crystals  and  microlites  in  the  ground- 


PYROXENE  ANDESITK.  363 

mass  of  thin  section  107  are  similarly  decomposed  and  colored  with  red  oxide  of  iron,  as 
in  the  corresponding  variety  of  andesite  from  Trail  Hill.  The  same  is  true  of  thin  sec- 
tion 108,  the  excess  of  red  oxide  rendering  the  slide  nearly  opaque.  The  augite  micro- 
lites  in  the  groundmass  are  very  abundant  and  are  traceable  directly  to  the  larger 
crystals;  they  are  in  stout  prisms  or  irregular  grains  and  in  most  every  case  have  one 
or  more  magnetite  grains  attached.  The  pyroxene  in  these  rocks,  like  that  in  the 
andesite  of  Richmond  Mountain  consists  of  pleochroic  hypersthene  and  nonpleochroic 
augite,  with  the  same  characteristic  differences  throughout. 

The  hornblende  is  much  less  abundant  than  the  pyroxene  and  qpcurs  only  in 
larger  phenocrysts,  with  poorly  denned  outline,  being  frequently  rounded  and  also 
irregular,  as  though  corroded.  The  cross  sections  are  six  and  occasionally  eight  sided, 
and  show  the  prism  and  piuacoids.  They  are  surrounded  by  a  heavy  black  border, 
the  substance  of  which  sometimes  penetrates  nearly  to  the  center  of  the  crystal.  A 
zonal  arrangement  of  the  minute  .magnetite  particles  is  seen  in  some  individuals,  thin 
section  107.  The  hornblende  is  brown,  with  strong  pleochroism :  c  =  dark  reddish 
brown,  b  =  brown,  a  =  light  brown,  c>b>a.  Inclusions  are  few,  except  grains  of 
magnetite,  beside  which  there  are  a  few  prisms  of  apatite  having  a  sharp  hexagonal 
cross  section. 

Biotite  phenocrysts  are  present  in  small  amount,  always  with  rounded  outlines 
and  crowded  with  magnetite  grains.  Magnetite  and  apatite  occur  as  in  the  Richmond 
Mountain  andesite.  Quartz,  though  quite  noticeable  in  macroscopic  grains  in  the 
hand  specimens  as  an  accessory  mineral,  is  not  found  in  the  thin  sections  studied, 
except  one  small  particle,  0-25 nun  in  diameter,  which  carries  both  glass  and  fluid  inclu- 
sions (107). 

The  groundmass  is  composed  of  feldspar  and  augite  niicrolites,  with  much 
minute  magnetite  associated  with  the  augite,  crowded  together  in  a  colorless  glass 
base,  the  whole  showing  a  distinct  flow- structure.  The  proportion  of  augite  and 
feldspar  is  about  equal,  but  the  size  of  the  microlites  is  not  so  uniform  as  in  the 
Richmond  Mountain  andesite,  and  numerous  crystals,  from  0-05  to  O-l"""  long,  arc 
scattered  through  the  mass,  giving  it  a  much  less  homogeneous  texture.  The  funda- 
mental structure,  however,  is  felt-like,  which  completes  the  correspondence  between 
the  two  pyroxene-andesites  of  the  district,  which  are  indeed  but  15  miles  apart. 
They  represent,  however,  a  rock  of  very  wide  occurrence  in  the  West,  judging  by  the 
collection  of  the  Exploration  of  the  Fortieth  Parallel,  which,  with  a  constant  micro- 
scopic habit  of  groundmass  and  of  phenocrysts,  varies  only  in  macroscopic  habit; 
that  is,  in  compactness,  structure,  and  color,  and  in  the  relative  size  or  abundance  of 
the  phenocrysts,  and  in  the  absence  or  presence  of  hornblende  and  biotite,  an  excess  of 
which  is  generally  accompanied  by  a  modification  of  the  groundmass,  resulting  in 
difficulty  deterimnable  forms  intermediate  between  pyroxeue-audesite,  hornblende- 
andesite,  and  hornblende-mica-andesite.  From  the  foregoing  description  it  is  evident 


364  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

that  the  rocks  forming  Richmond  Mountain  and  Cliff  Hills  are  pyroxene-andesites, 
with  a  very  considerable  percentage  of  hornblende  as  an  essential  ingredient  and 
biotite  as  an  accessory  one.  They  might  in  fact  be  termed  hornblende-pyroxeue- 
andesites. 

A  very  striking  correspondence  between  the  different  varieties  in  each  of  the 
two  pyroxene-andesite  occurrences  in  the  district  will  be  seen  on  comparing  together 
thin  sections  102,  104  with  77,  78,  79;  107  with  90,  and  108  with  92.  The  other  sec- 
tions from  Richmond  Mountain  have  corresponding  varieties  at  Cliff  Hills,  which, 
however,  were  not  made  into  thin  sections.  Several  thin  sections  remain,  which  need 
a  brief  mention.  No.  109  is  from  a  modification  of  the  Cliff  Hills  rock,  which  in  some 
respects  resembles  the  basalt  of  Magpie  Hill  and  that  on  the  south  slope  of  Alhambra 
Ridge,  but  which  is  found  under  the  microscope  to  be  a  much  finer  grained  pyroxene- 
andesite,  rich  in  magnetite,  with  phenocrysts  of  the  same  feldspar  and  pyroxene,  but 
without  hornblende  or  mica,  and  bearing  some  small  red  altered  crystals  of  olivine, 
whose  presence  might  throw  considerable  doubt  over  the  determination  were  there 
not  frequent  patches,  not  inclusions,  of  coarser  grained  groundmass  free  from  magnet- 
ite, identical  with  the  grouudmass  of  the  neighboring  andesite.  Thin  section  110  is 
a  variety  from  a  limited  exposure  in  the  tuff  northwest  of  Devils  Gate,  which  is  poor 
in  large  phenocrysts. 

Hornbiende-[Mica]-Andesite.— The  hornblende-[mica]-andesite  of  the  district,  though 
closely  related  to  the  pyroxene-andesite  in  many  respects,  has  sufficient  strongly 
marked  points  of  contrast  to  constitute  a  separate  rock.  The  areas  of  exposure  of 
the  two  in  the  field  are  nowhere  in  contact,  and  no  transition  of  one  into  the  other  is 
detected  under  the  microscope,  except  in  the  andesitic  pearlites  to  be  described  later. 
The  hornblende  [micaj-ande^ite  has  a  light  purple  and  reddish  purple  groundmass  rich 
in  macroscopic  crystals  of  feldspar,  hornblende,  and  biotite,  of  which  the  feldspar  pre- 
dominates; it  is  further  characterized  by  the  total  absence  of  pyroxene.  Several  mod- 
ifications which  occur  in  separate  and  limited  exposures  will  be  noticed  in  their  proper 
connection.  Under  the  microscope  the  rock  (thin  sections  38,  41.  35,  37,  42,  42a,  39) 
is  seen  to  consist  of  a  microcrystalline  feldspathic  groundmass,  in  some  cases  entirely 
free  from  glass  base.  It  is  rich  in  macroscopic  pheuocrysts  of  feldspar,  black  bordered 
hornblende,  and  reddish  brown  biotite.  The  accessory  minerals  are  apatite  and  very 
little  magnetite,  and  zircon;  quartz  is  an  accessory  mineral  in  some  occurrences,  espe- 
cially in  that  east  of  Pinto  Road  (39). 

The  feldspar  is  wholly  triclinic,  being  for  the  most  part  striated,  and  the 
unstriated  sections  giving  angles  of  extinction  belonging  only  to  plagioclase.  The 
porphyritical  crystals  are  beautifully  developed,  yielding  sharply  outlined  sections, 
one  or  two  millimeters  in  length,  of  the  usual  form.  They  show  remarkably  fine  zonal 
structure,  well  illustrated  in  Fig.  1,  PI.  v.  In  this  feldspar,  besides  the  successive 
stages  when  the  crystal  had  rectilinear  outlines,  there  were  three  periods  when  its 


HORNBLENDE  MICA -ANDESITE.  365 

form  must  have  been  quite  rounded  as  i  t  partially  fused.     The  cleavage  in  these  feld- 
spars is  very  imperfect,  and  is  for  the  most  part  wanting,  the  crystals  being  irregu- 
larly cracked  like  sanidine.     The  polysynthetic  twinning  after  albite  and  i>ericline  is 
very  unevenly  developed.     The  latter,  never  repeated  to  any  great  extent,  is  present 
in  many  individuals,  the  lamella;  seldom  traversing  the  entire  width  of  the  crystal; 
those  produced  by  the,  former  twinning  vary  greatly  both  in  breadth  and  length  in 
the  same  individual,  as  well  as  in  different  ones,  the  feldspars  in  general  being  char- 
acterized by  a  paucity  of  striations.     This  is  well  shown  iu  Figs.  3  and  4,  PL  v,  and 
Fig.  '2,  PI.  vi,  the  two  figures  on  PI.  v,  also  illustrating  the  characteristic  difference 
between  the  largest  of  the  phenocrysts  (Fig.  3,  PI.  v),  and  the  medium  sized  ones 
(Fig.  4,  PI.  v),  both  being  magnified  to  the  same  extent,  35  diameters.    The  largest 
have  quite  irregular  outlines  and  an  abundance  of  stria;,  while  the  medium  sized  feld- 
spars are  very  sharply  crystallized  and  are  poorly  striated.     Besides  the  multiple 
twinning,  nearly  every  individual  is  twinned  in  halves,  either  after  albite  or  in  a 
manner  corresponding  to  that  of  Carlsbad  in  orthoclase;  and  frequently  several  indi- 
viduals have  formed  in  parallel  orientation  with  the  brachypinacoid  as  the  plane  of 
contact  (Fig.  2,  PI.  vi).    The  angle  of  extinction  averages  about  the  same  in  each  of 
the  thin  sections  studied.    By  far  the  larger  number  of  readings  give  angles  ranging 
from  15°  to  31°,  some  being  lower  and  a  very  few  being  higher;  for  example,  in  thin 
section  35  the  observed  angles  in  the  zone  perpendicular  to  the  brachypinacoid  are 
7°,  150,  20°,  200,  210,  210,  280, 300,  31°,  320,  350,  350,  4<P.     In  the  other  thin  sections 
the  higher  angles  are  even  scarcer  and  belong  to  very  perfectly  rectangular  sections 
with  few  striations.     Anorthite  is  probably  present  only  in  small  amounts,  the  greater 
number  of  the  porphyritical  feldspars  being  labradorite.    The  lath-shaped  microlites 
of  feldspar  in  the  groundmass  are  fibrous  and  twinned,  and  have  angles  of  extinction 
varying  only  a  few  degrees  from  zero  and  are  for  the  most  part  oligoclase;  the  species 
of  the  ill  defined  patches  or  grains  of  feldspar  in  the  groundmass  is  optically  indeter- 
minable.   The  large  crystals  of  feldspar  contain  numerous  small  colorless  glass  inclu- 
sions, each  with  a  single  gas  bubble ;  what  sometimes  appear  like  particles  of  dust 
are  found  under  a  high  power  to  be  aggregates  of  these  sharply  defined  inclusions 
0- .  02mm  in  diameter ;  they  are  occasionally  arranged  iu  systematic  order,  but  more  com- 
monly are  scattered  irregularly  through  the  crystal;  some  are  attached  to  needles  of 
apatite.    There  are  also  inclosed  a  few  microlites  of  apatite,  some  of  which  in  turn 
contain  glass  inclusions,   and  rarely  small  crystals  of  zircon.     The  substance  of  the 
feldspar  in  the  thin  sections  studied  is  absolutely  fresh. 

The  hornblende  is  found  only  in  macroscopic  crystals  affording  the  character- 
istic six  sided  cross  sections  and  having  in  every  case  a  black  bonier,  except  in  the 
green  variety  of  hornblende-[mica|-andesite  at  the  east  base  of  Hoosae  Mountain  i4_'. 
42«).  The  substance  of  the  hornblende  is  entirely  decomposed,  not  a  single  unaltered 
fragment  having  been  found  in  any  of  the  thin  sections.  The  resulting  product  is  a 


366  GEOLOGY  OF  THE  EUEEKA  DISTRICT. 

fluely  fibrous,  yellowish  green  mineral,  faintly  pleochroic  and  with  a  rather  high  index 
of  refraction,  whose  angle  of  extinction  is  over  20°  in  some  sections,  and  which  must 
therefore  be  a  fibrous  amphibole.  Its  fibers  start  from  transverse  fissures  and  run 
parallel  to  the  vertical  axis  of  the  crystal  for  a  short  distance,  thus  forming  a  net 
work,  the  meshes  of  which  are  filled  with  a  colorless  substance  without  noticeable 
action  on  polarized  light,  who  ;e  granular  texture  and  botryoidal  form  suggest  amor- 
phous quartz.  Scattered  through  these  minerals  are  numerous  opaque  and  transpar- 
ent globulites  of  indeterminable  nature,  together  with  ferrite  and  red  oxide  of  iron 
directly  traceable  to  the  black  border,  from  which  it  projects  into  the  crystal  in  thin 
branching  plates  (35).  A  more  intimate  mixture  of  the  two  principal  decomposition 
products  has  taken  place  in  thin  section  41,  most  of  which  has  fallen  out  in  grinding. 
In  thin  sections  42,  42«,  the  hornblende  sections  are  without  black  borders  and  the 
alteration  has  advanced  farther,  resulting  in  a  yellowish  green,  fibrous  chlorite,  with 
extinction  parallel  to  the  fibers,  which  has  spread  through  the  groundmass  of  the 
rock,  imparting  to  it  a  green  color. 

Biotite  is  less  abundant  than  the  hornblende,  though  in  much  larger  crystals. 
It  occurs  in  six-sided  crystals  with  very  marked  pleochroism  and  strong  absorption, 
being  deep  red  when  the  section  is  parallel  to  the  base,  and  in  oblique  sections  in 
ordinary  light  orange,  yellow,  and  green  (35).  The  interference  figure  shows  a  small 
optical  angle,  which  varies  somewhat  in  different  thin  sections,  the  plane  of  the  optic 
axes  being  perpendicular  to  that  of  symmetry  in  the  crystal.  Twinning  parallel  to 
<x>P  (110),  where  the  composition  plane  is  the  base  OP,  is  frequent.  It  is  recognized 
by  bands  with  different  angles  of  extinction  in  sections  slightly  inclined  to  the  base, 
and  by  the  different  positions  of  the  interference  figures  in  such  sections,  and  also  by 
difference  in  the  pleochroism  in  some  sections  nearly  perpendicular  to  the  base. 
There  are  numerous  black  microlites  arranged  in  lines  perpendicular  to  the  six  faces 
of  the  mica  crystal,  besides  irregularly  scattered  prisms  of  apatite,  and  more  rarely 
zircon.  In  thin  section  39  there  are  portions  of  the  groundmass,  each  containing  one 
or  more  apatite  crystals.  The  biotite  has  remained  perfectly  fresh  in  most  of  the  thin 
sections,  though  the  hornblende,  has  been  entirely  decomposed.  In  section  37  the 
biotite,  though  bleached  out  and  stained  yellow  by  iron  oxide,  still  retains  its  optical 
properties. 

Quartz  appears  as  a  very  inconstant  accessory  ingredient,  being  wholly  wanting 
in  the  form  of  primary  phenocrysts  in  the  typical  crystalline  hornblende-[mica]-aude- 
site  41,  35,  but  occurring  in  abundance  in  the  glassier  variety  from  east  of  the  Pinto 
|{<i;nl  (39),  where  it  is  in  rounded  grains  and  fragments,  the  largest  3mm  in  diameter. 
They  carry  numerous  dihexahedral  glass  inclusions  with  gas  bubbles,  around  which 
in  polarized  light  the  quartz  shows  the  phenomena  produced  by  strain.  Other  small 
grains  and  fragments  are  found  sparingly  in  thin  sections  42«  and  38.  Microscopic 


HORNBLENDE  MICA-ANDES1TE. 

quartz  grains  form  a  constituent  of  the  groundmass  in  the  crystalline  forms  of  t  In- 
rock  and  are  determiuable  as  such  in  thin  section  41;  they  are  more  numerous  in  thin 
section  35.  They  also  occur  in  small  aggregates  around  the  sides  of  cavities  resem- 
bling chalcedony.  The  abundance  and  intimate  association  of  this  modification  of 
quartz  with  the  groundmass  of  the  rock,  and  the  abundance  of  macroscopic  quartz 
in  the  rock  near  Pinto  Road  makes  it  an  intermediate  variety  between  andesite  and 
(Incite. 

Magnetite  in  macroscopic  grains  and  in  microscopic  crystals  is  very  evenly  dis- 
seminated through  the  groundmass,  but  is  not  nearly  so  abundant  as  in  the  pyroxene 
andesite.  It  is  everywhere  coated  with  red  oxide  and  in  thin  section  37  it  has  been 
converted  into  the  yellow  hydrous  oxide. 

Apatite  is  especially  well  developed  in  stout  hexagonal  prisms  with  a  pyramidal 
termination  and  occasionally  the  base,  a  beautiful  example  being  found  in  thin  sec- 
tion 37,  Fig.  4,  PI.  in.  They  are  dusted  gray  in  the  center  and  show  the  customary 
pleochroism.  Cross  sections  show  inclusions  parallel  to  the  sides  of  the  prism. 
There  are  also  glass  inclusions  in  negative  crystals,  Fig.  1,  PI.  in.  Apatite  is  asso- 
ciated with  hornblende  and  biotite  and  also  occurs  isolated  in  the  groundmass;  it  is 
specially  noticeable  in  thin  section  37.  It  has  a  flue  red  color  in  thin  sections  35,  39. 
Zircon  is  a  constant  ingredient,  though  in  very  small  quantities.  It  is  in  microscopic 
crystals  of  a  yellow  color  easily  recognizable  by  their  sharp  outline,  high  index  of 
refraction,  and  consequent  brilliant  display  of  interference  colors  between  crossed 
nicols.  They  are  rather  more  frequent  in  thin  sections  35,  42. 

The  groundmass  of  the  typical  hornblende- f  mica] -andesite  of  this  district  (35,41) 
is  microcrystalline  without  glass.  It  is  composed  of  microlites  of  plagioclasc,  largely 
oligoclase,  in  an  aggregate  of  feldspar  and  quartz  grains  of  irregular  outline,  that  are 
nearly  free  from  microlites  ac  the  center,  especially  in  thin  section  41.  Besides  these 
minerals  are  minute  crystals  of  magnetite  and  in  thin  section  35  opaque  microlites, 
which  are  seen  to  be  made  up  of  opaque  aud  transparent  yellow  grains  and  correspond 
to  the  shreds  of  brown  mica  that  occur  in  thin  section  41.  This  is  more  abundant  in 
the  fine  grained  audesitic  breccia  (38),  where  it  also  occurs  in  well  defined  hexagonal 
plates.  A  flow  structure  is  evident  in  the  arrangement  of  the  lath-shaped  feldspar 
microlites.  The  groundmass  in  thin  sections  42,  42a,  39  presents  a  less  advanced 
stage  of  crystallization,  the  lath-shaped  microlites  being  accompanied  by  smaller  and 
fewer  faintly  polarizing  feldspar  grains  in  a  relatively  small  amount  of  colorless  glass. 
Through  this  in  the  green  variety  from  the  east  base  of  Hoosac  Mountain  (42,  42a)  is 
disseminated  yellowish  green  fibrous  chlorite,  resulting  from  the  decomposition  of  tin- 
hornblende. 

Thin  sections  45,  46,  and  48  are  from  highly  decomposed  rock,  \yhose  original 


368  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

character  has  wholly  disappeared,  but  which  still  retains,  besides  some  partially  altered 
mica,  apatite  and  zircon  in  a  perfectly  fresh  condition. 

An  interesting  example  of  altered  hornblende-mica-audesite  and  at  the  same 
time  of  local  accessions  of  porphyritical  quartz  is  the  occurrence  west  of  Glen  Dale 
Valley,  south  of  Hoosac  Mountain  (49, 50).  The  feldspar  of  the  rock  is  completely 
replaced  by  calcite,  quartz  and  colorless  potash-mica,  which  are  also  the  residual 
products  of  the  decomposition  of  the  grouudmass.  Hornblende  with  dark  border  is 
very  abundant  in  thin  section  49.  It  is  altered  to  yellowish  green,  coarsely  fibrous 
chlorite,  which  polarizes  strongly  and  extinguishes  parallel  to  the  fibers,  and  contains 
small,  yellow,  highly  refracting  grains,  possibly  epidotc.  The  mica  has  become  color- 
less, but  retains  its  negative,  apparently  uniaxial  character,  and  is  rich  in  the  most 
beautifully  developed  microscopic  crystals,  that  are  in  part  tetragonal  pyramids  of  a 
colorless  mineral  with  high  index  of  refraction,  apparently  anatase,  in  part  slender 
prisms  of  epidote  ( ?)  and  thin  plates  of  hematite,  besides  smaller  grains  in  lines  perpen- 
dicular to  the  sides  of  the  mica  plate  and  apatite  prisms  with  glass  inclusions  (49). 
Quartz  is  sparingly  present  in  rounded  macroscopic  grains  in  the  variety  rich  in  horn- 
blende, but  is  very  abundant  in  the  purple  variety  (50),  poor  in  hornblende,  which  in 
the  field  appears  as  a  local  modification  of  the  former.  The  quartz  occurs  both  in 
rounded  fragments  and  in  dihexahedral  crystals  and  contains  glass  inclusions". 

Andesitic  Pcariite  and  Dacite.— The  third  and  most  interesting  form  of  andesite  found  in 
the  district  unites  under  its  numerous  and  varied  forms  characters  both  of  the 
pyroxeue-andesite  and  hornblende-mica-audesite,  and  presents,  as  an  extreme 
variety,  daeite.  Its  connection  is  most  intimate  with  the  hornblende-uiica-andesite, 
which  indeed  is  found  passing  into  it  in  an  outcrop  back  of  the  windmill  pump  east 
of  Secret  Canyon  road  (74),  and  also  east  of  Hoosac  Mountain  (73,  75a,  75b,  76).  It 
is  again  found  in  association  with  liornblendevinica-andesite  in  Sierra  Canyon  and  in 
the  gulch  south  of  Carbon  Ridge.  Its  resemblance  to  pyroxene-andesite  will  be  seen 
to  be  confined  to  a  variety  with  inicrolitic,  felt-like  groundmass,  and  to  the  presence 
of  pyroxene,  which  is  lacking  in  the  hornblende-mica-andesite  just  described.  In  the 
two  principal  localities  where  it  has  come  to  the  surface,  at  Dry  Lake  and  in  the 
vicinity  of  Sierra  Canyon  and  South  Hill,  it  presents  so  great  a  variety  of  form  that 
the  extremes  of  the  series  would  scarcely  be  suspected  of  belonging  to  the  same  geo- 
logical body,  but  this  is  evidently  the  case  in  the  occurrence  south  of  Dry  Lake.  The 
following  able  of  comparison  is  arranged  to  show  the  parallelism  in  the  forms  from 
which  thin  sections  have  been  made.  The  series  commences  with  the  most  crystalline 
variety  and  that  most  clossly  allied  to  the  horublende-mica-andesite,  and  finishes 
with  the  most  glassy,  porous,  and  quartzose  form,  or  dacite 


ANUESIT1C  PEARLITE  AND  DACITE. 


Dry  Lake. 

Sierra  Canyon. 

South  of  Carbon 
Ridge. 

Kast  of  Hiin-:if 
Mountain. 

52 

53 
54 

55 

61 

62      63 

71 

73 
74    75  a    75  b 

56 

57 

64 
66 

58 

68 

76 

69 

59 

70 

60 

The  rock  in  question  is  a  pearlite  with  a  variously  modified  glassy  grouudinass 
and  abundant  phenocrysts  of  feldspar,  hornblende,  hypersthene,  augite,  biotite  and 
quart/.  The  character  of  these  minerals  is  constant  throughout  and  similar  in  most 
respects  to  those  found  in  the  more  crystalline  andesites.  They  may  be  here  described 
then  for  the  whole  series.  The  feldspar  is  triclinic,  eight-tenths  of  the  individuals 
being  striated  and  the  remainder  giving  angles  of  extinction  for  twins  in  the  zone  at 
right  angles  to  the  composition  plane  too  great  for  orthoclase.  In  only  a  very  few 
individuals  was  the  nature  of  the  feldspar  in  doubt  and  its  orthotomic  nature  possi- 
ble, but  even  here  the  evidence  was  entirely  negative.  It  is  probable  that  orthoclase 
is  present  to  a  very  limited  extent — one  macroscopic;  crystal,  a  carlsbad  twin  in  hand 
specimen  51,  a  hornblende-mica-andesite  from  Sierra  Canyon  like  52,  having  a 
brilliant  unstriated  basal  cleavage,  proved  optically  to  be  sanidiue.  The  crystal  form 
of  the  plagioclase  is  like  that  already  described,  but  a  greater  number  are  in  angular 
fragments.  The  zonal  structure  is  marked  and  the  polysyuthetic  twinning  irregular; 
in  many  instances  the  stria?  are  scarcely  perceptible,  and  in  a  few  cases  they  are 
altogether  wanting.  The  angles  of  extinction  range  from  a  few  degrees  to  between 
25°  and  31°.  The  feldspars  are  therefore  labradorite  in  part,  though  a  large  propor- 
tion are  probably  andesine.  In  several  thin  sections  they  correspond  to  anorthite, 
which  will  be  more  fully  noticed  under  the  description  of  the  different  varieties.  The 
inclusions  of  glass  and  of  the  grouudmass  are  the  same  as  those  in  the  feldspar  of 
the  pyroxene-andesite  and  hornblende-mica-audesite. 

The  hornblende  is  without  a  dark  border  of  any  kind.  It  is  in  all  other 
respects  like  that  found  in  the  pyroxene-andesite  and  the  fresher  hornblende  mica- 
andesite  of  Sierra  Canyon  (52).  It  occurs  in  well  developed  crystals  of  a  greenish 
brown  color  with  very  strong  pleochroism.  There  are  no  characteristic  inclusions, 
it  being  for  the  most  part  quite  free  from  them.  It  appears  to  have  withstood  decom- 
position better  than  the  hypersthene,  as  it  shows  no  signs  of  alteration  even  in  juxta- 
position to  almost  completely  altered  hypersthene.  Fig.  5,  PI.  in.  The  hyi>ersthen«' 
MON  XX 24 


370  GEOLOGY  OF  THE   EUREKA  DISTRICT. 

is  strongly  pleochroic,  green  and  ligbt  reddish  brown,  similar  in  all  points  to  that  of 
the  pyroxene-andesite.  Its  decomposition,  which  is  the  same  as  that  already  described, 
has  advanced  farther  than  in  the  pyroxene-andesites  and  is  illustrated  in  Figs.  5  and 
!>,  PI.  in.  Augite  is  found  only  in  two  thin  sections  from  this  locality,  Nos.  56  and  57. 

Biotite  is  macroscopically  the  most  prominent  mineral  in  the  quartzose  members 
of  this  series.  It  is  in  hexagonal  plates  of  a  dark  brown  color  with  strong  absorption, 
and  is  optically  negative  with  a  very  small  angle  between  the  optic  axes.  It  is 
twinned  as  in  the  hornblende-mica-andesite.  Quartz  is  not  so  abundant  in  the  thin 
sections  as  in  the  hand  specimens  and  is  always  in  rounded  grains  or  angular  frag 
ments  with  a  few  glass  inclusions.  Magnetite,  apatite,  and  zircon  are  common  to  all 
the  varieties  of  this  pearlite.  The  apatite  is  like  that  found  in  the  other  audesites;  a 
fine  example  showing  the  terminations  and  a  basal  cleavage  is  represented  in  Fig.  8, 
PI.  in. 

The  zircon  crystals  are  not  more  numerous  in  this  than  in  many  other  rocks 
where  there  are  found  to  be  three  or  four  crystals  to  a  rock  section,  but  their  occur- 
rence here  in  unaltered  feldspars  or  isotropic  glass  renders  them  more  than  usually 
favorable  for  study,  and  so  a  number  have  been  drawn  to  show  their  crystal  faces, 
which  were  recognized  by  careful  study  in  all  possible  lights  and  were  drawn  with  the 
aid  of  a  camera.  Owing  to  the  high  index  of  refraction  of  zircon  the  marginal  faces 
can  not  be  as  accurately  determined  as  those  near  the  center  of  the  figures,  and  the 
terminal  planes  of  Figs.  15  and  20  being  extremely  minute  could  not  be  made  out  for 
the  same  reason.  It  should  also  be  remarked  that  the  drawings  are  not  mathematical 
projections,  because  with  the  high  magnifying  power  employed,  in  one  instance  900 
diameters,  only  a  small  part  of  a  crystal  is  in  focus  at  any  one  time,  and  a  certain 
amount  of  distortion  necessarily  follows.  The  figures  represent,  however,  the  sharp- 
ness of  the  crystallization  and  will  indicate  the  forms  taken  by  the  crystals.  Besides 
the  short,  stout  crystals,  from  0-05  to  ()•!  """  long,  more  usually  met  with,  there  are 
sometimes  long,  slender  prisms  reaching  a  length  of  0-37  mm  and  terminated  at  one  or 
both  ends,  Figs.  15  and  16,  PI.  in.  The  form  of  Fig.  15  appears  to  be  the  two  prisms, 
ooP,  ooPoo  ,  the  double  pyramid  or  zirconoid  3P3,  and  the  pyramids  P  and  Pec  ;  that 
of  Fig.  16  ooP,  cePx. ,  and  3  P3;  and  Fig.  17  o>P,  ooPoo  ,  3  P3,  with  P  or  .P»  .  Fig. 
18  represents  a  very  simple  form,  combining  a  prism  with  a  pyramid  of  the  opposite 
order.  Fig.  19  seems  to  present  both  prisms  xP,  -»Poo  ,  the  double  pyramid  3P3, 
and  the  two  pyramids  P  and  P-«  ;  and  Fig.  20  ooP,  ooPao  ,  3P3,  and  two  pyramids  P 
and  Pec  .  The  occurrence  of  similar  microscopic  zircons  has  been  observed  by  the 
writer  in  most  all  kinds  of  rocks,  except  the  very  basic,  but  more  especially  in  the 
mica-bearing  varieties,  with  which  mineral  it  is  frequently  in  close  association. 

In  noticing  the  different  varieties  of  this  audesitic  pearlite  the  description  will 
be  confined  to  the  series  found  in  the  vicinity  of  Dry  Lake  and  the  correspondence  or 
points  of  difference  in  the  similar  forms  from  the  other  localities  will  be  mentioned  in 


ANDES1T1O  PEAKL1TE  AND  DACITE.  371 

their  proper  connection.  At  the  top  of  the  table  stands  the  quartz-bearing  horn- 
blende-mica-andesite  (52)  found  in  Sierra  Canyon,  forming  the  connecting  link  that 
unites  by  its  microscopic  structure  the  hornblende-mica-andesites  and  andesitic 
pcarlites.  The  groundinass  of  this  rock  is  completely  crystalline,  exactly  as  in  the 
typical  hornblende-mica-andesite  of  the  district  (35).  In  the  thin  section  besides  the 
plagioclase  there  are  two  or  three  unstriated  sections  which  may  possibly  belong  to 
sauidiue.  The  fresh  hornblende  is  without  dark  border,  a  few  individuals  having  a 
slight  aggregation  of  magnetite  grains  around  them,  which  is  also  noticeable  around 
the  biotite.  There  is  no  pyroxene  present,  but  some  well  developed  quartz  crystals. 
The  nearest  approach  to  crystalline  andesite  in  the  Dry  Lake  series  is  thin  section 

53,  whose  gray  grouudmass  is  microspherulitic.     The  spherulites  are  composed  of 
radiating  colorless  needles,  besides  which  are  multitudes  of  transparent  globulites 
and  trichites,  straight  and  curved,  some  black  and  opaque,  others  red  and  referable 
to  mica,  and  some  formed  of  a  string  of  transparent  grains  which  are  also  found  in 
short,  stout,  interpenetrating  microlites,  which  appear  to  belong  to  augite.     The 
whole  shows  a  marked  flow  structure  and  bears  phenocrysts  of  labradorite,  biotite 
and  hornblende  crowded  with  magnetite  grains  and  no  longer  fresh;  besides  com- 
pletely altered  pyroxene  [hypersthene] ;  zircon  occurs  in  good  crystals.    There  is  no 
macroscopic  quartz,  but  small  aggregations  of  colorless  plates  appear  to  be  tridymite. 
Thin  section  61  is  more  highly  crystalline  and  illustrates  the  first  stages  of  the  forma- 
tion of  the  feldspathic  grains  in  the  groundmass  of  the  hornblende-mica-audesite;  they 
are  seen  forming  around  the  phenocrysts  as  centers,  which  are  the  same  as  those 
in  53  with  the  addition  of  macroscopic  quartz. 

A  modification  common  to  four  separate  localities  is  represented  by  thin  section 

54,  and  approaches  closely  to  the  pyroxene-andesite  of  the  district;   the  silver  gray 
grouudmass  has  a  satin-like  sheen  in  transmitted  light,  produced  by  fibrous  feldspar 
microlites  in  nearly  parallel  arrangement  in  a  colorless  glass  base,  having  a  marked 
flow  structure,  with  a  felt-like  appearance  in  the  thicker  parts  of  the  section ;  there 
are  also  grains  of  magnetite  and  a  little  hyperstheue.    The  larger  phenocrysts  are 
well  developed  and  the  inclusions  are  very  fine.     Feldspar  is  in  excess  of  the  other 
constituents,  and   hornblende  and   hyperstheue  occur  in  about  equal  proportions, 
biotite  being  scarce.    The  corresponding  varieties  (62,  63,  71,  73)  are  almost  identical. 
In  62  the  feldspar  microlites  are  more  delicate,  biotite  is  wanting  and  quartz  occurs 
in  macroscopic  grains;  63  is  richer  in  glass  and  poorer  in  large  crystals  and  has  a 
little  brown  mica  in  the  groundmass. 

In  71  the  glassy  grouudmass  is  richer  in  augite  microlites,  and  also  contains  some 
of  hornblende  and  biotite.  It  very  closely  resembles  the  pyroxeue-andesite  of  Rich- 
mond Mountain;  73  is  remarkable  for  the  abundance  of  biotite  in  hexagonal  plates  in 
the  groundmass.  This  variety  of  the  pearlite  is  further  characterized  by  the  presence 
of  feldspars  with  very  high  angles  of  extinction,  several  of  which  reach  40  and  4f>°, 


372  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

indicating  auorthite,  which  is  the  feldspar  so  abundant  in  the  pyroxene-andesite. 
The  next  variety  is  a  still  more  glassy  rock,  55;  it  is  a  colorless  glass  with  a  pearlitic 
fracture,  with  scattered  microlites,  which  are  beautifully  developed,  some,  in  long 
prisms  with  pyramidal  termination  and  transverse  jointing,  appear  to  be  apatite; 
others,  shorter  and  stouter,  are  more  doubtful,  but  resemble  those  in  53,  which  are 
probably  augite.  There  are  also  curved  and  tapering  microlites  and  strings  of  grains 
apparently  of  the  same  mineral.  Larger  microscopic  crystals  scattered  through  the 
groundmass  are  hypersthene,  hornblende  and  biotite.  There  is  but  little  magnetite. 
Of  the  macroscopic  crystals,  feldspar  is  very  abundant  as  labradorite,  with  possibly  a 
little  anorthite;  biotite  is  also  abundant,  and  hornblende  and  hyperstheue  are  scarce. 
There  is  one  rounded  grain  of  quartz  with  good  rhombohedral  cleavage.  Thin  section 
75  is  like  55;  75«  is  taken  from  the  ^ame  specimen  and  shows  a  slight  modification 
caused  by  streams  of  opaque  particles  and  hair-like  trichites,  which  lie  scattered  or 
aggregated  in  the  most  delicate  dendritic  forms.  A  small  part  is  black  in  incident 
light  and  may  be  magnetite,  but  the  greater  part  is  bright  red  and  is  hematite.  Thin 
section  74  is  similar. 

The  remaining  varieties  differ  from  the  preceding  in  having  the  glass  base  tilled 
with  opaque  and  more  or  less  transparent,  ill  denned  microlites  and  flocculent  matter, 
imparting  to  it  a  black,  led,  yellow  or  white  color.  Thin  section  56  is  from  a  brecciated 
pearlite  rich  in  angular  fragments  and  crystals  of  labradorite,  hypersthene,  augite, 
hornblende,  and  magnetite,  with  no  mica.  The  groundmass  is  glass,  probably  o 
itself  colorless,  but  so  crowded  with  microlites  and  more  or  less  opaque  grains  as  to 
appear  in  the  section  dark  brown,  yellow,  and  bluish.  In  some  places  it  is  brown  and 
globulitic,  in  others  it  is  tilled  with  flocculent  matter,  which  is  brown  in  transmitted 
light  and  white  in  incident;  in  other  places  it  is  colorless,  with  few  microlites.  The 
transition  from  one  kind  to  another  is  generally  sudden  and  the  flow  structure  is  well 
^narked,  being  especially  beautiful  in  thin  section  57,  which  is  similar  to  56,  as  is  also 
64,  though  of  a  lighter  color.  -Mica  and  quartz  are  both  wanting  in  these  last  three 
thin  sections.  The  varieties  represented  by  thin  sections  58  and  66  are  very  similar 
to  the  last,  much  more  so  than  their  appearance  in  the  hand  specimen  would  indicate. 
Their  secretions  are  the  same — labradorite,  hyperstheue,  and  hornblende,  with  a  little 
augite  and  no  mica  or  quartz.  They  are  not  brecciated,  however,  and  the  groundmass 
is  lighter  colored,  the  opacite  being  red  and  white  in  incident  light  and  the  flow 
structure  very  striking.  There  is  a  tine  example  of  partially  altered  hypersthene 
shown  in  Fig.  9,  PI.  in. 

Variety  59  has  a  more  pumice-like  groundmass,  the  glass  having  numerous  gas 
bubbles.  It  is  much  lighter  colored,  with  more  white  and  less  yellow  opacite,  and  is 
in  part  cryptocrystalline.  The  pheuocrysts  are  labradorite,,  eight-tenths  of  the  feld- 
spars showing  stria}  and  the  rest  probably  belonging  to  plagioclase.  There  is  a  very 
little  partially  altered  hypersthene,  considerable  hornblende,  and  much  biotite.  The 


ANDES1TIC   PKAHLITK  AND  DACITE.  373 

quartz,  so  abundant  in  the  hand  specimen  in  large  rounded  grains,  is  scarce  in  the 
thin  section.  The  two  varieties  from  Sierra  Canyon  (68,  69)  are  denser  than  that  just 
described.  The  glass  groundmass  of  68  is  without  gas  bubbles  and  is  crowded  with 
yellowish  translucent  particles,  which  reflect  incident  light  and  appear  white.  It  is 
in  places  spherulitic  and  abounds  in  angular  fragments  of  plagioclase,  nine-tenths  of 
the  feldspars  being  striated.  There  is,  besides,  quartz,  with  fine  glass  inclusions,  a 
very  little  pyroxene,  more  hornblende,  and  much  biotite.  Thin  section  69  is  identical 
with  the  last  under  the  microscope.  Thin  section  76,  from  east  of  Hoosac  Mountain, 
is  similar  to  the  foregoing,  but  has  a  cryptocrystalline  gronndmass  and  is  somewhat 
decomposed.  Some  portions  of  the  groundmass  of  70  are  crystalline  and  bear  feld- 
spar microlites,  but  the  whole  is  the  same  as  08  and  69.  Thin  section  60  is  more 
porous,  but  has  the  characteristics  of  the  last  four  sections.  Its  feldspar  is  all  plagio- 
clase and  gives  angles  of  extinction  corresponding  to  labradorite.  This  last  quartz- 
bearing  group  (59,  60,  68,  69,  70)  appears  to  be  true  dacite.  and  as  such  is  very 
interesting. 

It  may  be  well  to  note  at  this  point  some  of  the  characteristic  features  distin- 
guishing these  closely  allied  rocks  as  they  are  found  in  this  district.  The  gronndmass 
of  the  hornblende-mica-andesite  is  in  general  microcrystalline,  without  glass,  having, 
besides  lath-shaped  feldspar  microlites,  which  are  probably  oligoclase,  interpenetrating 
grains  of  quartz  and  feldspar.  It  is  freer  from  magnetite  and  contains  no  pyroxene. 
The 'groundmass  of  the  pyroxene-andesite,  on  the  other  hand,  is  very  glassy,  with  a 
felt-like  structure  produced  by  feldspar  and  augite  microlites,  the  feldspar  being 
labradorite,  with  an  abundance  of  magnetite.  The  pheuocrysts  of  the  former  rock 
are  labradorite,  dark  bordered  hornblende  in  every  case  decomposed,  considerable 
biotite,  and  sometimes  quartz,  but  no  pyroxene  or  the  remains  of  any.  The  pheno- 
crysts  of  the  pyroxene-andesites  are  auorthite,  hypersthene,  augite,  dark  bordered 
hornblende,  with  very  little  biotite  and  only  an  occasional  quartz.  The  andesitic 
pearlites  hold  an  intermediate  position  between  the  two,  some  of  the  varieties  being 
quite  like  the  horublende-mica-andesite,  while  others  approach  closely  to  pyroxene- 
andesite,  yet  all  have  features  differing  from  both.  The  groundmass  is  a  glass  more 
or  less  full  of  microlites,  and  in  the  greater  number  of  cases  is  crowded  with  indeter- 
minable globulites  and  pai-ticles.  Besides  the  feldspar  phenocrysts,  which  are  for  the 
most  part  labradorite  and  possibly  a  very  little  orthoclase,  with  some  anorthite,  there 
are  hornblende  .crystals  without  dark  border,  hypersthene,  a  little  angite,  biotite,  and 
quartz.  The  dacites  are  a  modification  in  which  the  macroscopic  quartz  has  greatly 
increased,  together  with  the  biotite,  while  pyroxene  has  nearly  disappeared.  They 
are  also  the  most  pumice-like. 


CHAPTER    III. 

RHYOLITE. 

There  are  three  distinct  varieties  of  rhyolite  iii  the  Eureka  District,  more  notice- 
ably distinct  iii  the  hand  specimen  than  in  thin  section,  since  their  essential  constitu- 
ents are  the  same  throughout.  The  difference  arises  from  a  change  in  the  relative 
proportion  of  the  phenocrysts  and  in  the  nature  of  the  groundmass.  That  from 
about  Pinto  Peak  which  covers  the  greatest  area  has  a  light  colored  groundmass, 
for  the  most  part  white,  also  gray  and  purplish  gray,  partly  vitreous  and  partly 
crystalline  in  appearance,  with  numerous  porphyritical  crystals  of  quart/  and  feld- 
spar and  a  few  scattered  bits  of  mica.  A  second  variety,  from  Rescue  Canyon,  has  a 
reddish  purple,  vitreous  groundmass,  crowded  with  large  crystals  of  quart/  and  bril- 
liantly reflecting  sauidine;  and  the  third,  from  south  of  Carbon  Ridge,  has  a  dense, 
reddish  purple  groundmass,  often  finely  banded,  having  few  phenocrysts  except  those 
of  copper-colored  mica.  Upon  a  superficial  examination  of  these  rocks  in  the  field  it 
would  seem  natural  to  separate  the  three  varieties  into  the  classes  suggested  by  Von 
Richthofen  in  1867.'  That  from  Rescue  Canyon  has  all  the  appearance  "at  a  dis- 
tance" of  granite,  and  might  be  said  to  be  "granite-like,"  while  that  from  Pinto  Peak 
is  certainly  "  porphyry-like,"  and  the  variety  from  south  of  Carbon  Ridge,  being 
quite  poor  in  macroscopic  crystals  and  having  a  beautifully  banded  structure,  answers 
to  the  description  of  rhyolite  proper;  but  under  the  microscope  the  granite-like  variety 
is  found  to  have  an  almost  wholly  glass  groundmass,  and  to  correspond,  therefore, 
more  or  less  closely  to  quartz-porphyry.  The  groundmass  of  the  porphyry-like  kind, 
on  the  contrary,  is  found  to  be  microcrystalline  in  most  cases,  or  microgranitic,  and 
the  third  to  vary  from  a  quite  glassy  to  an  entirely  crystalline  rock.  Hence  no  sys- 
tematic classification  has  been  undertaken,  the  varieties  receiving  local  designations 
sufficient  for  the  purposes  of  the  present  report.2 

pinto  Peat  Rhyolite.— Under  the  microscope  thin  sections  from  a  great  number  of  speci- 
mens of  this  variety  present  an  extremely  monotonous  appearance ;  a  fine  grained, 
more  or  less  wholly  crystalline  groundmass  rich  in  large  crystals  and  fragments  of 


1  Von  Richthofen   Natural  System  of  Volcanic  Bocks,  San  Francisco.  1867,  p.  16. 

2Since  this  was  written  u  study  of  the  rhvolites  of  the  Great  Basin  led  to  more  definite  conclusions  regarding  von 
Kichthofen's  classification  of  rhyolites,  which  were  expressed  in  a  paper  on  the  volcanic  rocks  of  the  Great  Kasin  by  Arnold 
Hague  and  J.  P.  Iddings.  Am.  Jour.  Sri.,  vol.  xxvn  1884,  p.  461. 

374 


RHYOLITK.  :>,7;, 

quartz  jintl  feldspar,  with  occasionally  a  little  biotite.  The  microscopical  habit  of  the.se 
porphyritical  crystals  is  so  constant  in  all  the  thin  sections  of  this  group  as  to  permit 
of  a  single  detailed  description,  the  different  modifications  of  the  groundmass  only 
requiring  special  notice.  The  feldspar  present  is  sauidine,  with  which  plagioclase  is 
associated  to  a  greater  or  less  extent.  The  latter  is  in  some  cases  entirely  wanting, 
but  in  others  is  almost  as  abundant  as  the  sanidine.  Sometimes  both  occur  in  very 
small  quantities  in  the  thin  sections  and  hardly  ever  outnumber  the  quartz.  Sani 
dine  occurs  in  well  developed  crystals  and  also  in  angular  fragments.  Sections  of 
the  former  are  mostly  rectangular,  with  the  corners  rounded;  others  show  more  than 
four  sides  and  indicate  that  their  crystal  form  is  made  up  of  OP,  ooPoc,  ooP,  2P* 
Zonal  structure  is  rarely  observed.  Many  of  the  individuals  are  in  Carlsbad  twins. 
The  cleavage  is  frequently  very  perfect,  though  often  entirely  wanting,  but  there 
are  always  concoidal  fractures,  and  the  resemblance  to  quart/,  is  often  very  striking, 
requiring  an  optical  test  to  distinguish  between  them.  It  is  characterized  by  a  much 
lower  double  refraction,  which  in  these  extremely  thin  sections  causes  it  to  remain 
generally  dark  or  but  faintly  lighted  between  crossed  nicols.  Quite  a  number  of 
sections  happen  to  be  nearly  at  right  angles  to  the  optical  bisectrix  and  exhibit  very 
small  angles  between  the  optic  axes,  the  interference  figure  being  almost  a  cross  and 
showing  the  bisectrix  negative.  There  are  several  of  these  in  thin  section  112.  A 
fortunate  section  parallel  to  the  clinopinacoid  occurs  in  thin  section  142  and  is  at 
right  angles  to  the  optical  normal,  which  is  found  to  be  positive,  the  interference  figure 
being  hyperbolas  that  unite  in  the  center  of  the  field.  The  inclination  of  the  plane  of 
the  optic  axes  is  about  +  7°  to  the  basal  cleavage,  and  the  angles  of  the  sides  of  the 
feldspar  section  correspond  to  those  cut  from  0  P,  ocP,  and  2P  oc.  Besides  the  basal 
cleavage,  which  in  this  section  is  very  perfect,  is  a  second  less  regular  cleavage 
parallel  to  the  trace  of  the  orthopinacoid.  The  plane  of  the  optic  axes  in  these  sani- 
diues  is  sometimes  in  the  plane-  of  symmetry,  sometimes  at  right  angles  to  it.  The 
substance  of  the  sanidine  is  very  pure  and  free  from  inclusions  of  foreign  matter. 
Numerous  minute  gas  cavities,  however,  occur  irregularly  scattered,  some  of  which  have 
their  sides  wet  with  fluid,  but  the  gas  has  always  the  greater  volume.  A  notable 
exception  to  this  freedom  from  inclusions  occurs  in  thin  section  141,  Fig.  2,  PI.  v, 
where  two  sharply  outlined  crystals  of  sauidine  grown  together  with  different  orien- 
tation about  a  fragment  of  plagioclase  are  filled  with  quartz  in  orderly  arranged 
forms,  with  constant  crystallographic  orientation  throughout  certain  portions  of  the 
feldspar  crystals,  which  is  shown  by  the  extinction  of  light  and  the  parallel  position  of 
numerous  small  dihexahedral  glass  inclusions  with  gas  bubbles,  found  only  in  the 
quartz,  whilst  irregularly  shaped  gas  cavities  occur  in  the  feldspar  substance.  This 
is  a  most  interesting  fact  from  its  relation  to  the  subject  of  fluid  and  glass  inclusions 
in  volcanic  rocks,  for  it  would  appear  from  this  instance  that  quartz  and  feldspar  crys- 
tallizing out  at  the  same  time  and  under  the  same  conditions  have  inclosed,  the  one 


37B  GEOLOGY  OF  THE  EUitEKA  DISTRICT. 

glass  with  gas,  the  other  gas  without  glass,  which  gas  appears  in  the  larger  cavities 
to  be  associated  with  water,  suggesting  that  its  condition  at  the  time  of  its  inclosure 
was  that  of  highly  expanded  steam.  This  crystal  is  further  interesting  as  a  sporadic 
development  of  micropegmatitic  structure. 

The  plagioclase  is  in  crystals  very  similar  to  those  of  sanidine,  but  is  not  nearly 
so  abundant,  being  almost  entirely  wanting  in  all  of  the  thin  sections  from  the  rhyolite 
dikes  (Nos.  138,  140,  141,  142,  144, 143,  146, 153,  155,  148, 150, 134,  137).  The  twinned 
lamellae  vary  considerably  in  length,  breadth  and  frequency,  and  in  most  all  the  indi- 
viduals are  twinned  both  after  the  law  of  albite  and  that  of  pericline,  besides  which 
the  composite  crystals  are  also  twinned  in  a  manner  corresponding  to  the  Carlsbad 
twins  of  orthoclase,  which  can  be  seen  from  the  outline  of  the  section  and  inequality 
of  the  sets  of  angles  of  extinction  in  the  two  halves,  Fig.  7,  PI.  in.  The  investi 
gation  of  the  extinction  angles  was  not  very  satisfactory,  owing  to  the  scarcity  of 
favorable  sections;  the  majority  of  readings  were  low,  the  highest  being  17°,  leading 
to  the  conclusion  that  the  triclinic  feldspar  is,  for  the  most  part,  oligoclase.  It  is  also  ' 
of  very  pure  substance,  with  few  gas  cavities  and  more  rarely  small  glass  inclusions;  it 
is  without  zonal  structure  and  has  poorly  marked  cleavage.  The  feldspar  is  extremely 
fresh  in  the  thin  sections  from  the  region  of  Pinto  Peak  and  in  those  from  most  of  the 
dikes,  but  is  partially  replaced  by  calcite  and  kaolin  in  thin  section  146.  In  138  it  is 
entirely  altered  to  calcite  and  kaolin,  the  latter  appearing  in  the  thin  section  as  a 
colorless  aggregate  of  fibrous,  faintly  polarizing  particles. 

The  most  abundant  and  constant  of  all  the  ingredients  is  quartz,  the  pheno- 
crysts  of  which  are  well  developed  dihexahedrons  and  angular  fragments,  less  fre- 
quently rounded  grains.  It  is  irregularly  cracked  and  of  very  pure  substance,  free 
from  inclusions,  except  an  occasional  "bay"  of  gronndmass  and  a  few  colorless  glass 
inclusions  with  single  gas  bubble,  around  which,  in  some  cases,  is  seen  in  polarized 
light  the  phenomena  of  strain  or  unequal  tension,  the  effects  of  which  are  still  further 
shown  by  small  cracks  that  pass  through  the  center  of  the  dihexahedral  glass  inclu- 
sions and  extend  a  short  distance  into  the  quartz  crystal,  constituting  three  planes 
corresponding  to  three  of  the  planes  of  symmetry  parallel  to  the  vertical  axis.  These 
appear  in  longitudinal  section  as  a  straight  line  or  an  inclined  fracture,  and  in  cross 
section  as  a  six  rayed  star.  A  flue  illustration  is  found  in  thin  section  111,  Figs.  1 
and  2,  PI.  IV,  where  a  cross-section  and  longitudinal  section  occur  within  1"""  of 
one  another.  In  the  cross- section  of  quartz  is  a  minute  fluid  inclusion  with  moving 
bubble,  a  very  rare  occurrence,  though  quite  numerous  fluid  inclusions  are  found  in 
the  fine  dihexahedrons  of  quartz  in  thin  section  127.  Quartz  in  irregular  grains  forms 
a  large  part  of  the  ground  mass.  Small  phenocrysts  of  biotite  are  found  sparingly  in 
some  of  the  thin  sections,  but  are  wholly  wanting  in  others.  The  biotite  is  for  the 
most  part  free  from  magnetite  grains  or  other  inclusions  when  fresh.  It  is  altered  in 
some  cases  to  a  colorless,  brilliantly  polarizing  mica,  crowded  with  yellow,  opaque 


HHYOLITK.  377 

grains.  This  group  of  rhyolites  is  very  poor  in  accessory  minerals,  there  being  only 
two,  which  are  of  exceptional  occurrence.  Zircon  in  fragments  and  minute  crystals 
is  occasionally  met  with  in  association  with  biotite.  Garnet  in  well  developed 
dodecahedrons,  and  also  in  irregular  grains  of  a  light  red  color  in  thin  section,  occurs 
in  NOB.  Ill,  112,  122,  and  123. 

The  most  striking  feature  ot  this  variety  of  rhyolite  is  its  groundmass,  which 
presents  the  micrograuitic  structure,  not  frequently  met  with.  The  remarkable  thin- 
ness of  the  sections  prepared  from  this  rock  offers  a  highly  satisfactory  field  for  study 
and  leaves  no  reasonable  doubt  of  the  entire  absence  of  glass  in  the  composition  of 
the  grouudmass  of  most  of  the  thin  sections.  Besides  the  granular  crystalline  develop- 
ment there  are  those  that  are  partly  cryptocrystalline  and  others  that  are  spherulitic 
and  glassy.  A  microcrystalliue  structure  is  common  to  thin  sections  111,  112,  113, 
114,  115,  116,  120,  119,  123,  140,  141,  153,  127,  134,  137.  The  groundmass  of  112  may 
be  taken  as  representing  that  of  all  the  first  nine  sections.  It  is  composed  of  micro 
scopic  interpenetrating  grains  of  quartz  and  feldspar,  through  which  are  scattered 
larger  grains,  averaging  0-06 """  in  diameter,  for  the  most  part  quartz,  with  gas  cavities 
like  those  in  the  phenocrysts  of  feldspar.  A  small  portion  is  deterininable  as  ortho- 
clase  and  striped  plagioclase.  The  quartz  is  often  in  aggregates  of  half  a  dozen  or 
more  grains  and  is  accompanied  by  irregular  fragments  of  light  red  garnet.  There  is 
also  a  little  biotite  in  microscopic  crystals,  more  abundant  in  thin  section  116. 
Through  it  all  are  innumerable  dust-like  particles,  dark  in  transmitted  light,  but 
reflecting  incident  rays  and  giving  a  whitish  gray  color  to  the  section.  They  are 
probably  minute  gas  cavities.  In  addition  to  this  are  patches  of  yellow,  ill  defined 
grains,  corresponding  to  Vogelsang's  ferrite,  which  is  only  in  small  quantities  and 
alone  indicates  the  flow  structure,  best  seen  in  the  thin  section  without  the  aid  of  a 
lens.  The  groundmass  in  this  section  (112)  is  porous  and  is  filled  with  small,  irregularly 
shaped  cavities.  In  the  others  it  is  more  or  less  dense  and  varies  somewhat  in  the 
size  of  the  grains. 

Still  more  interesting  are  the  changes  of  structure  in  the  groundmass  of  the 
rhyolite  from  the  dikes.  Thin  sections  140,  141,  127,  134,  and  137  represent  the  most 
crystalline  variety,  being  coarser  grained  than  that  just  described.  They  are  without 
any  sign  of  flow  structure  and  carry  larger  grains,  which  are  micropegmatitic  in  tliiu 
section  140.  The  grains  are  composed  of  a  colorless  grain  or  crystal  of  quartz  with 
hexagonal  outline,  inclosing  semi-opaque  particles,  which  are  white  in  incident  light, 
and  are  sometimes  arranged  radially.  The  same  structure  appears  as  a  narrow 
border  around  the  quartz  phenocrysts.  The  grains  in  the  groundmass  of  thin  section 
141  are  also  mottled  in  polarized  light,  but  in  127,  where  the  similarly  clouded  grains 
attain  a  diameter  of  0-O.V"1",  one  in  the  thinnest  edge  of  the  section  shows  a  beauti 
fully  developed  micropegmatitic  structure,  which  near  the  center  of  the  grain  is  in  tri 
angular  figures  only  0-002 """  in  size,  and  near  the  edge  is  in  long,  narrow  strips.  The 


378  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

groundmass  of  138  and  146  consists  of  somewhat  larger  grains  of  quartz  in  a  crypto- 
crystalline  matrix,  which  is  identical  with  the  substance  occupying  the  sections  of 
decomposed  feldspars,  already  described  as  kaolin.  Here,  also,  it  is  possibly  the 
alteration  product  of  feldspar  in  the  groundmass  and  not  a  devitrified  glass.  The 
abundance  of  calcite  is  undoubtedly  due  to  infiltration  from  the  surrounding  limestone. 
Thin  sections  142,  143,  and  125  present  less  coarsely  crystallized  varieties,  the  first 
two  being  similarly  decomposed. 

A  lower  stage  of  crystallization,  in  which  the  groundmass  is  largely  or  entirely 
glass,  is  found  in  thin  sections  118,  155,  122,  117,  126,  and  130,  some  of  which  are 
partly  crystalline.  The  glass  is  spherulitic.  There  are  also  narrow  bands  of  fibers, 
the  fibers  lying  at  right  angles  to  the  direction  of  the  bands,  which  make  the  flow- 
structure  very  pronounced,  Fig.  1,  PI.  viu.  Thin  section  155  is  interesting  as  containing 
round  and  oval  spherules  of  colorless  glass,  with  a  few  concentric  inclusions,  remind- 
ing one  strongly  of  leucite  crystals.  They  polarize  faintly  in  radiating  rays.  An 
entirely  glassy  modification,  which  occurs  in  a  small  chimney  about  ten  feet  wide,  is 
shown  in  thin  sections  144, 145,  and  is  a  pale  green  glass  rich  in  feldspar  microlites, 
some  of  which  are  striated.  They  are  partly  rectangular,  with  the  four  corners  pro- 
longed like  a  "skate's  egg."  The  corners  of  others  are  fringed,  but  the  majority 
appear  like  bundles  of  colorless  fibers,  the  larger  of  which  are  compact  in  the  middle 
and  extinguish  light  as  a  single  individual,  Fig.  14,  PI.  in.  One  can  thus  trace 
the  connection  from  the  single  microscopic  fiber  to  the  dense,  sharply  crystallized 
feldspar,  that  is  large  enough  to  be  seen  without  the  use  of  a  lens.  In  the  thin  sec- 
tion, from  the  buff-colored,  porcelain- like  portion  of  the  same  flow  (145),  the  microlites 
are  more  numerous  and  are  accompanied  by  clouds  of  yellow  spots  with  aggregate 
polarization. 

Thin  sections  129  and  132  are  from  rhyolitic  pearlites,  poor  in  microlites,  with  some 
gas  cavities  and  globulites  of  an  indeterminable  nature.  The  pearlitic  structure, 
consisting  of  spherical  fractures  which  inclose  one  another  like  the  imbricated  scales 
of  an  onion,  is  very  well  marked  in  thin  section  129.  The  rhyolite  at  the  head  of 
Yahoo  Canyon  (157)  is  similar  to  the  eryptocrystalline  forms  of  this  variety  and  is 
poor  in  phenocrysts.  That  from  the  saddle  northeast  of  Combs  Mountain  (158)  cor- 
responds to  the  more  coarsely  microcrystalline  kind  and  is  very  poor  in  macroscopic 
crystals,  which  are  quartz,  feldspar,  and  biotite,  the  only  inclusions  noticed  being 
glass. 

A  completely  decomposed  rhyolite,  thin  section  148,  is  worth  mentioning.  In  the 
hand  specimen  it  is  seen  to  be  kaolin,  with  numerous  quartz  crystals.  In  thin  section 
it  is  colorless,  the  groundmass  having  no  action  whatever  ou  polarized  light  and 
being  filled  with  minute  grains  which  are  white  in  incident  light,  also  larger  yellow 
and  red  grains  of  iron  oxide,  resulting  apparently  from  the  decomposition  of  magnet- 
ite, besides  other  small  transparent  yellow  globulites  with  high  double  refraction, 


HHYOLITK.  379 

whose  nature  is  indeterminable.  The  macroscopic  quartz  crystals  have  colorless 
glass  inclusions,  which  are  for  the  most  part  spherical,  a  few  having  the  form  of  nega- 
tive crystals.  There  are  no  fluid  inclusions  and  some  of  the  quartzes  show  distinct 
rhombohedral  cleavage. 

Rescue  canyon  Rhyoiite.-The  second  variety  of  rhyolite  found  in  the  district,  thin 
sections  162,  165,  has  many  points  of  resemblance  in  microscopical  habit  to  that  just 
described.  It  is,  however,  richer  in  phenocrysts,  which  under  the  microscope  are 
found  to  be  angular  grains  and  fragments  of  quartz,  which  are  very  free  from  inclu- 
sions except  a  few  glass  dihexahedrons,  the  dark  color  of  the  quartz  not  being  trace- 
able to  noticeable  inclusions.  There  is  also  faintly  polarizing  sanidine,  sometimes 
indistinguishable  from  quartz  except  by  optical  tests.  In  this,  also,  there  are  no 
inclusions  to  account  for  the  slight  opalescence  seen  in  the  crystals  on  surfaces  at 
right  angles  to  the  base.  Besides  sanidine  there  is  a  comparatively  large  amount  of 
striated  plagioclase,  some  with  angles  of  extinction  corresponding  to  labradorite.  In 
addition  to  these  abundant  and  larger  phenocrysts  and  a  small  amount  of  biotite,  in 
which  this  variety  of  rhyolite  resembles  that  first  described,  there  is  a  small  per- 
centage of  pyroxene  in  fragments  and  crystals,  partly  altered;  one  or  two  fragments 
of  brown  hornblende  without  dark  border,  and  some  larger  magnetite  grains.  There 
is  also  an  irregular  grain  of  garnet  and  one  of  allanite.  The  groundmass  is  partly 
crystalline,  partly  glassy  and  axiolitic,  with  much  ferrite  in  fine  particles  which  mark 
its  fluidal  structure  and  give  it  a  red  color. 

Banded  Rhyolite.— The  third  variety  differs  from  both  the  others  and  is  in  some 
instances  of  rather  doubtful  nature,  owing  to  the  abundance  of  plagioclase  and 
scarcity  of  macroscopic  quartz.  The  four  thin  sections  prepared,  174,  173,  169,  1(58, 
have  numerous  points  of  resemblance  and,  though  differing  somewhat,  maybe  classed 
as  the  same  rock  and  described  as  rhyolites.  Thin  section  168  is  of  a  wholly  crystal 
line  rock,  in  which  the  phenocrysts  are  quartz  (with  a  few  glass  inclusions  and  less 
frequently  gas  cavities)  and  feldspar,  the  greater  part  of  which  is  sanidine,  which  is  with 
difficulty  distinguished  from  quartz  except  by  optical  tests.  Several  sections  of  saui- 
dine,  with  quadratic  form  and  right-angled  cleavage,  remain  dark  when  revolved 
between  crossed  nicols,  and  give  interference  figures  like  crosses  that  are  optically 
negative.  They  have  numerous  irregularly  shaped  gas  cavities,  which  are  especially 
abundant  near  the  margin  of  the  crystal.  Some  of  the  cavities  have  a  thin  coating 
of  fluid  around  their  walls,  and  a  few  contain  more  liquid  than  gas.  In  these  the 
bubble  is  movable.  There  is  also  plagioclase  and  a  little  biotite,  the  latter  tilled  with 
magnetite  and  red  oxide  of  iron.  The  groundmass  is  composed  of  quartz  grains,  un- 
striated  feldspar,  and  microscopic  spherulites,  with  many  curved  microlites  which  con- 
sist of  strings  of  transparent  grains  with  a  rather  high  index  of  refraction.  Besides 
these  there  is  a  little  mica  and  magnetite.  Thin  section  169  is  similar  in  the  char 
actor  of  its  groundmass,  which,  however,  is  less  coarsely  crystalline  and  has  a  more 


380  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

marked  flow  structure,  produced  by  variations  in  microstractnre  and  in  the  minute 
particles  of  coloring  matter.     It  is  poor  in  phenocrysts. 

The  two  remaining  thin  sections  (173,  174)  are  of  somewhat  doubtful  character. 
The  groundmass  is  but  partially  crystalline,  with  yellow  and  colorless  glass.  It  is 
rich  in  grains  of  iron  oxide,  both  black  and  red,  and  has  a  markedly  banded  structure, 
as  shown  in  Fig.  2,  PI.  viii.  It  is  poor  in  phenocrysts,  the  greater  number  being 
plagioclase,  with  marked  zonal  structure.  They  carry  more  glass  inclusions  than  are 
found  in  the  plagioclase  of  the  other  rhyolites  of  the  district.  Biotite  filled  with  red 
oxide  of  iron  is  next  in  abundance,  besides  which  there  is  a  little  apatite  in  compara- 
tively large  crystals,  and  one  crystal  of  augite.  There  are  numerous  groups  of  color- 
less grains  of  irregular  shape,  which  appear  to  be  tridymite.  There  is,  indeed,  a 
close  resemblance  in  some  of  its  microscopical  characters  to  certain  forms  of  andesite, 
while  at  the  same  time  it  seems  closely  allied  to  some  forms  of  rhyolite. 

Rhyoiitic  Pumice.— Before  describing  the  pumices  it  will  be  interesting  to  notice 
the  rhyolite  of  Purple  Hill  because  of  its  easily  traced  connection  with  the  adjoining 
pumice  and  pearlite  into  which  it  is  seen  to  pass.  Thin  section  176  is  from  rhyo- 
lite on  the  summit  of  the  hill ;  No.  177  is  from  the  same  at  the  northeast  base  of  the 
hill  where  it  passes  into  pumice.  No.  180  is  from  denser  pumice,  almost  pearlite,  and 
178  is  from  the  dark  compact  pearlite.  The  first  is  light  gray  in  thin  section  and  has 
a  glassy  groundmass  filled  with  faintly  polarizing  particles  and  larger  feldspar  micro- 
lites,  together  with  numerous  amygdules  of  tridymite.  The  phenocrysts  are  quartz 
and  feldspar,  of  which  sanidine  predominates  over  the  plagioclase.  There  is  a  little 
impure  biotite  and  a  fragment  of  pyroxene  and  some  magnetite.  In  the  next  thin  sec- 
tion (177)  the  phenocrysts  are  much  scarcer  and  the  groundmass  is  a  colorless  glass 
filled  with  gas  cavities,  some  of  which  are  spherical,  but  the  majority  are  elongated, 
spindle  shaped,  and  drawn  out  to  long  tubes,  that  are  much  twisted  and  bent.  There 
are  numerous  six-sided  microscopic  mica  plates  and  a  smaller  number  of  feldspar  and 
hornblende  microlites.  Much,  if  not  all,  of  the  opaque  grains  that  are  scattered  in 
patches  through  the  groundmass  is  foreign  to  the  rock  and  has  filled  cavities  during 
the  grinding  of  the  thin  section.  A  more  advanced  stage  is  seen  in  thin  section  180; 
the  phenocrysts  are  the  same  in  character,  but  are  more  abundant,  with  a  noticeable 
amount  of  pleochroic  hypersthene.  The  glassy  groundinass  is  rich  in  spherical  gas 
bubbles  and  microlites  of  feldspar,  hornblende,  and  biotite,  with  a  small  percentage  of 
trichites,  which  reach  a  greater  development  in  the  more  perfect  pearlite,  thin  section 
178.  The  colorless  glass  of  this  rock  bears  a  multitude  of  the  most  beautiful  micro- 
lites, consisting  of  colorless  rectangular  crystals  of  feldspar,  brown  hexagonal  plates 
of  biotite,  dark  green  prisms  of  hornblende,  and  curved  trichites  which  appear  opaque 
under  a  low  magnifying  power,  but  are  found  to  consist  of  a  transparent  fiber  with 
serrated  edges  or  to  be  a  string  of  disconnected  globulites.  They  are  grouped  about 


RHYOLITIC  PUMICE.  381 

an  opaque  grain  from  which  they  radiate  in  all  directions,  frequently  resembling  the 
down  of  a  thistle  and  suggesting  in  some  instances  a  bunch  of  ravelings.  There  are, 
besides,  other  indeterminable,  smaller  microlites  and  a  few  gas  bubbles. 

Closely  related  to  the  rhyolite  of  Purple  Hill,  both  in  their  field  occurrence  and 
mineral  composition,  and  in  the  latter  respect  allied  to  the  purple  rhyolite  of  Rescue 
Canyon,  are  the  tuffs  and  pumice  found  in  the  vicinity  of  the  town  of  Eureka,  on  the 
west  of  Richmond  Mountain,  and  also  on  the  south  slope  of  the  same  mountain. 
These  are  specially  interesting  because  of  numerous  alteration  products  which  have 
resulted  from  outflows  of  basalt  that  have  broken  through  them.  Thin  sections  from 
a  scries  of  specimens  representing  different  stages  of  alteration  naturally  exhibit  the 
same  character  of  phenocrysts,  which  have  not  been  affected  by  the  remelting  and 
may  therefore  be  considered  in  one  general  description,  the  modifications  of  the  pumice 
having  been  confined  to  the  glass  base.  Thin  sections  185,  188, 189, 193,  191,  192,  196, 
are  from  the  quarry  and  hill  slope  east  of  the  town.  The  phenocrysts  consist  of  augu 
lar  fragments,  seldom  of  perfect  crystals  of  quartz  and  feldspar  with  a  small  amount 
of  hypersthene,  hornblende,  and  biotite,  together  with  magnetite,  apatite,  zircon,  and 
garnet  as  occasional  accessory  minerals.  The  quartz  is  of  very  pure  substance  carry- 
ing only  glass  inclusions,  one  of  which,  in  thin  section  192,  is  brown,  a  neighboring 
inclusion  being  colorless.  There  are  two  instances  in  the  same  thin  section  of  quartz 
and  feldspar  grown  together  with  micropegmatitic  structure.  Of  the  feldspar,  sani- 
dine  is  the  predominating  species,  many  of  the  unstriated  sections  being  optically  deter- 
minable  as  such.  Triclinic  feldspar  is  always  present  in  greater  or  less  amount. 
A  zonal  structure  is  frequent  and  some  individuals  bear  inclusions  of  glass  in  the  form 
of  the  most  beautifully  defined  negative  crystals;  the  feldspar  is  everywhere  per- 
fectly fresh.  The  strongly  pleochroic  hypersthene  is  in  places  crowded  with  apatite 
and  glass  inclusions.  It  is  precisely  similar  to  that  in  the  pyroxene  andesite  and  ande- 
sitic  pearlites;  while  the  dark  green  hornblende  without  black  border  and  the  biotite 
correspond  exactly  to  the  same  minerals  in  the  pearlite.  The  accessory  minerals  ha  ve 
also  similar  characters  to  those  found  in  the  pearlites.  Small  fragments  of  allanite 
are  abundant  in  Nos.  189,  191,  192. 

Thin  sections  185  and  188  are  from  unaltered  portions  of  the  pumice  breccia; 
185  is  from  the  quarry  back  of  the  engine  house  and  188  from  a  spot  6  feet  distant 
from  the  plane  of  contact  with  the  basalt  a  little  to  the  north.  They  are  essentially 
the  same  rock,  188  being  the  better  section.  It  consists  of  a  fine  grained  mixture  of 
colorless  pumice  fragments  full  of  elongated  fluid  inclusions  with  variously  sized  gas 
bubbles,  sometimes  looking  like  welded  glass  threads,  together  with  a  projwrtionately 
smaller  amount  of  crystallized  minerals,  in  a  matrix  of  yellow  glass  that  appears  to  be 
made  up  of  minute  glass  particles  held  together  by  glass,  in  which  are  much  fewer 
gas  cavities,  and  which  is  partly  cryptocrystalline.  There  are  also  occasional  frag- 
ments of  glass  of  other  kinds,  some  brown  and  others  microfelsitic  and  in  part  crypto- 


382  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

crystalline,  the  outline  of  the  brecciated  fragments  being  sharp  and  well  defined. 
Thin  section  189,  taken  from  a  spot  18  inches  distant  from  the  line  of  contact,  shows 
the  effects  of  partial  remelting,  the  character  and  composition  of  the  breccia  being  the 
same  as  in  the  last  thin  section.  The  greatest  change  is  noticed  in  the  colorless 
porous  fragments,  where  the  size  of  the  fluid  and  gas  cavities  has  been  greatly 
reduced,  the  whole  seeming  to  be  contracted  and  crumpled  together;  there  begin  to 
appear  also  in  the  place  of  the  cavities  minute  black  grains  and  microlites  in  small 
numbers.  The  definition  of  the  pumice  fragments  is  no  longer  marked,  and  they 
commence  to  merge  in  the  surrounding  matrix. 

In  thin  sections  193, 191,  and  192,  from  immediate  contact  with  the  basalt,  where 
the  fusion  has  been  complete,  the  resulting  body  is  a  compact  glass  almost  free  from 
gas  or  fluid  inclusions,  which  have  been  driven  out  by  the  heat,  since  the  mass  was 
under  little  or  no  pressure.  The  glass  in  some  instances,  as  in  section  192,  has  retained 
its  former  brecciated  character,  preserving  the  outline  of  its  component  fragments,  but 
has  so  contracted  as  to  present  many  more  phenocrysts  to  the  same  area  of  thin  section 
and  has  become  of  very  dark,  blue-black  color.  This  color  seems  to  be  due  to  innumer- 
able black  hair-like  trichites,  opaque  grains,  and  a  smaller  number  of  transparent 
microlites,  both  short  and  stout  and  long  and  curved.  There  are  in  this  thin  section 
portions  of  the  neighboring  basalt  having  an  exceptionally  dark  brown  glass  base.  In 
193  and  191  the  evidence  of  a  former  brecciation  has  almost  entirely  vanished ;  the  glass 
of  the  different  fragments  in-  some  places  has  been  very  uniformly  mingled,  especially 
in  193,  though  occasional  fragments  have  offered  greater  resistance  to  fusion.  The 
lighter  color  of  193  is  due  to  the  reflection  of  light  from  mist-like  clouds  of  gas  bubbles 
of  the  minutest  dimensions,  which  appear  at  first  to  be  opaque  particles,  but  are  found 
under  a  power  of  850  diameters  to  be  transparent  globules,  with  a  heavy  dark  border. 
They  are  especially  abundant  around  two  small  cavities  in  thin  section  191  and  prob- 
ably cause  the  yellowish  white  lining  of  the  larger  cavities  in  the  hand  specimen, 
which  is  peculiar  to  several  occurrences.  Thin  section  196  is  from  another  form  of 
alteration  of  the  same  pumice ;  it  is  rather  more  crystalline  and  is  filled  with  opaque 
particles  that  are  red  and  yellow  in  incident  light  and  give  the  rock  its  color;  it  is 
also  very  porous. 

The  same  effects  have  been  produced  in  the  pumice  by  the  numerous  outbreaks 
of  basalt  along  the  south  slope  of  Richmond  Mountain,  and  the  thin  sections  from 
this  locality  present  in  many  instances  the  same  characters  as  those  just  described. 
They  will  therefore  need  but  a  brief  mention  and  will  serve  rather  as  evidence  of  the 
identity  of  the  two  bodies  of  pumice  and  of  the  uniformity  of  the  alteration  arising 
from  the  same  cause.  Thin  sections  199,  200,  204,  205,  206,  207,  208,  and  209  are  from 
rocks  on  the  small  spur  south  of  the  summit  of  the  mountain,  and  occurring  under 
different  conditions  they  vary  somewhat  in  character.  Thin  section  19!)  is  of  a  fine 
grained  altered  pumice  not  in  immediate  contact  with  basalt,  and  resembles  thin 


EHYOLITIC  PUMICE. 

section  196.  There  is  inucb  opaque  coloring  matter  in  the  base  and  an  abundance  «»f 
phenocrysts  consisting  of  much  quart/  and  nearly  equal  quantities  of  sanidine  and 
triclinic  feldspar;  in  one  Carlsbad  twin  one  half  exhibits  an  interference  cross  that  is 
optically  negative,  while  the  other  half  gives  a  bar  parallel  to  the  clinopinacoid,  in 
which  case  the  section  must  be  perpendicular  to  the  negative  bisectrix  of  the  first 
half,  having  an  angle  between  the  optic  axes  of  about  0°,  and  at  the  same  time  at 
nearly  right  angles  to  one  of  the  optic  axes  in  the  other  half  having  a  large  optical 
angle  and  the  plane  of  the  axes  parallel  to  that  of  symmetry.  Still  another  Carlsbad 
twin  shows  the  plane  of  the  optic  axes  normal  to  that  of  symmetry.  There  is  rather 
more  hypersthene  than  is  common  to  these  pumices.  It  is  partially  decomposed  and 
displays  a  very  striking  pleochroism,  owing  to  the  thickness  of  the  section. 

Thin  section  200  is  the  most  interesting  of  all  the  alteration  products,  on  ac- 
count of  its  undoubted  relations  to  the  basalt  and  its  higher  degree  of  metamor- 
phism ;  it  is  traceable  directly  to  the  same  deposit  of  pumice  as  199,  and  lies  in  apparently 
undisturbed  layers  directly  over  basalt,  which  did  not  in  this  instance  reach  the  sur- 
face, but  thoroughly  altered  the  overlying  pumice,  breaking  through  it  lower  down 
the  slope.  In  thin  section  it  is  a  whitish  gray,  fine  grained  breccia  of  about  the  same 
grain  as  199.  Under  the  microscope  the  porphyritical  crystals  are  seen  to  be  angular 
fragments  of  quartz,  sanidine,  and  plagioclase  of  the  same  size  and  abundance  as 
those  in  the  last  named  section;  pyroxene,  however,  is  wanting  and  only  a  little 
biotite  is  present,  besides  a  single  grain  of  garnet.  The  groundmass  has  retained  its 
brecciated  character,  though  the  pumice  fragments  have  lost  their  original  form  and 
appear  to  merge  into  one  another;  but  the  degree  of  crystallization  is  far  more 
advanced,  hardly  any  portion  of  it  being  without  influence  on  polarized  light.  As  a 
natural  result  of  its  brecciated  character  the  structure  is  most  varied,  which  is  the 
more  pronounced  between  crossed  nicols.  It  is  partly  sphernlitic  and  axiolitic  and 
partly  cryptocrystalline  and  in  places  it  is  microcrystalline  in  irregular  grains. 

Thin  sections  204  and  205  are  from  a  small  outbreak  of  rhyolite  on  the  south 
side  of  the  spur  about  100  yards  from  the  locality  of  199,  which,  though  not  traced  in  the 
field  to  unaltered  pumice,  exhibits  under  the  microscope  so  close  a  resemblance  in  many 
respects  to  the  last  described  form  as  to  leave  little,  if  any,  doubt  that  this  small  flow 
of  porcelain-like  rhyolite  is  a  highly  altered  pumice  breccia  that  has  escaped  from  its 
place  of  confinement,  probably  having  been  heated  under  pressure  to  a  greater  degree 
than  the  breccia  met  with  in  situ  on  the  surface.  In  thin  section  it  is  whitish  gray. 
204  having  a  glassy  groundmass  strongly  resembling  that  of  199,  which  is  filled  with 
faintly  polarizing  particles,  and  shows  as  great  a  diversity  of  structure,  which  indi- 
cates its  once  brecciated  condition.  It  is  in  places  spherulitic,  cryptocrystalline,  and 
microcrystalline.  There  is  a  marked  flow  structure  and  a  smaller  amount  of  frag- 
mentary crystals,  consisting  of  quartz  and  feldspar,  with  very  little  biotite  and  one 
fragment  of  greenish  brown  hornblende.  Thin  section  20-">  has  a  gronmlmass  of  more 


;;s4  (UROLOGY  or  THE  EUKEKA  DISTRICT. 

uniform  structure,  composed  of  microscopic  grains  of  varying  size,  which  pass  into 
cryptocrystalline  portions.  The  flow  structure  is  most  noticeable  in  the  thin  section 
without  the  aid  of  a  lens.  The  phenocrysts  are  quartz  and  feldspar  in  fragments.  A 
cross  section  of  zircon,  oi1""'  broad,  shows  only  one  set  of  prism  faces  and  a  good 
cleavage  parallel  to  the  other,  which  is  seldom  met  with  in  microscopic  zircon  crystals 
(Fig.  10,  PI.  III.)  This  thin  section  is  similar  to  those  from  the  Pinto  Peak  rhyolite. 

Thin  sections  206,  207,  and  208  from  contact  with  basalt  on  the  slope  and  at 
the  base  of  the  same  spur  show  exactly  the  same  kind  of  alteration  as  193,  191,  and 
192.  which  have  been  already  described.  The  glass,  however,  is  brown  and  red,  with- 
out the  black  trichites,  and  there  is  only  a  trace  of  the  bisilicates  and  of  biotite.  No. 
209  is  a  beautiful  section  of  a  reddish  brown  breccia  formed  of  fragments  of  brown 
glass  almost  free  from  microlites  cemented  together  by  a  dark  red  ferrite-bearing 
glass,  rich  in  microscopic  shreds  of  biotite,  which  is  very  abundant,  together  with 
green  hornblende  in  small  fragments.  Pyroxene  is  scarce,  there  is  comparatively 
little  quartz,  and  there  are  about  equal  amounts  of  sanidine  and  plagioclase,  besides 
which  are  magnetite  garnet  and  zircon. 

Of  the  remaining  instances  of  altered  pumice  one  from  contact  with  basalt  on 
the  end  of  the  east  spur  of  Hornitos  Cone,  210,  is  of  purplish  brown  glass,  containing 
portions  with  very  different  structures,  being  itself  an  intimate  mixture  of  brown  and 
gray  glass  with  numerous  grains  of  magnetite  and  a  great  abundance  of  brown  horn- 
blende in  fragments,  and  with  more  perfect  crystals  of  strongly  pleochroic  hypers- 
thene  with  a  narrow  dark  border;  besides  biotite,  quartz,  and  feldspar,  of  which 
plagioclase  is  in  excess.  The  relative  amount  of  the  bisilicates  and  mica  is  much 
greater  than  in  any  of  the  pumices  previously  described,  and  with  an  excess  of 
triclinic  feldspar  approaches  nearer  to  the  composition  of  an  andesite.  The  altered 
breccia  from  the  summit  of  the  cone  presents  in  thin  section  211  a  reddish  gray  matrix, 
bearing  yellow,  orange,  and  red  fragments,  which  are  found  to  vary  greatly  in  micro- 
structure.  There  are  comparatively  few  and  small  phenocrysts,  principally  of  quartz 
and  feldspar,  with  still  less  biotite.  The  groundmass  is  a  glass,  in  places  microfelsitic, 
also  spherulitic,  and  passing  from  cryptocrystalline  into  microcrystalline.  Among  the 
fragments  are  several  that  appear  to  belong  to  basalt. 

Similar  to  the  last  is  the  coarse  breccia  from  the  east  side  of  Black  Canyon,  three 
sections  of  which  exhibit  the  changes  wrought  by  the  adjacent  basalt.  In  general 
they  are  poor  in  phenocrysts,  plagioclase  being  the  most  abundant,  together  with  a 
little  biotite  and  pyroxene,  and  besides  the  variously  modified  glassy  portions  are  pieces 
of  the  same  basalt.  The  glass  of  thin  section  224  is  filled  with  irregularly  shaped  fluid 
inclusions  with  stationary  bubbles,  besides  patches  of  gray  polarizing  particles  and 
numerous  magnetite  grains.  In  thin  section  225  the  fluid  inclusions  have  diminished 
both  in  size  and  number  and  the  contorted  flow  structure  of  the  individual  glass  frag 
meuts  has  been  reduced  more  nearly  to  straight  lines  and  to  a  general  parallelism 


EHYOLITIC  PUMICE. 

throughout  the  whole  mass,  except  in  the  case  of  the  less  fusible  pieces.  The  fluid 
inclusions  have  wholly  disappeared  from  the  glass  of  thin  section  L'L'G,  which  is  both 
colorless  ;i  nd  bright  yellow,  and  is  full  of  opaque  red  particles,  without  doubt  red  oxide 
of  iron.  It  is  rich  in  trichites  and  microlites  of  feldspar,  some  of  which  are  colored 
yellow. 

Prom  the  foregoing  it  appears  that  the  richly  quartzose,  rhyolitic  pumice  in  tin- 
vicinity  of  Richmond  Mountain,  containing,  as  it  does,  a  large  percentage  of  triclinic 
feldspar,  which  is,  however,  subordinate  in  amount  to  the  monoclinic,  and  at  the  same 
time  carrying  a  varying  amount  of  biotite,  pyroxene,  and  green  hornblende,  holds 
an  intermediate  position  mineralogically  between  the  dacite  and  the  rhyolite  of  Res- 
cue Canyon. 

A  thin  section  of  pumice,  241,  altered  to  a  compact  glass  by  the  rhyolite  of  Pinto 
Peak,  is  interesting  as  containing  only  a  little  mica  in  addition  to  the  quartz  and 
feldspar,  and  therefore  closely  resembling  in  composition  the  surrounding  rhyolite. 
In  addition  to  these  phenocrysts,  which  are  few,  is  garnet.  The  glassy  groundmass 
is  nearly  colorless,  and  contains  only  a  small  amount  of  black  particles  and  starlike 
groups  of  trichites.  Another  altered  pumice,  242,  from  the  basin  west  of  Secret 
Canyon  road,  is  like  the  last  in  composition,  the  light  brown  glass  being  in  places 
filled  with  rectangular  microlites  of  feldspar. 

Differing  greatly  from  the  foregoing  pumices  is  a  tuff  of  fine  grain  occurring  over 
a  small  area  on  the  east  slope  of  Hornitos  Cone,  where  it  appears  as  a  bedded  deposit 
of  dark  gray  volcanic  sand,  altered  by  an  outflow  of  basalt  to  a  blue  black,  basalt- 
looking  mass.  Thin  section  223  shows  it  to  consist  of  a  purplish  brown  glass  crowded 
with  fragments  of  feldspar,  hypersthene,  and  augite,  with  some  black  bordered  horn- 
blende and  large  grains  of  magnetite.  The  feldspar  is  wholly  tricliuic,  the  angle  of 
extinction  in  several  instances  exceeding  that  of  labradorite  and  corresponding  to 
anorthite.  It  also  contains  a  multitude  of  colorless  glass  inclusions  and  a  few  large 
ones  of  brown  glass.  The  hypersthene  has  the  pleochroism  common  to  that  of  the 
neighboring  andesite,  and  the  greenish  brown  hornblende  fragments  are  all  sur- 
rounded by  a  black  border.  There  is  no  doubt  that  this  tuff  belongs  to  pyroxene- 
andesite.  though  it  is  the  only  occurrence  of  the  kind  met  with  in  the  district.  The 
brown  glass  is  in  places  globulitic,  with  more  or  less  feldspar  microlites  and  black 
grains  and  trichites  in  very  beautiful  aggregations. 
MON  xx 25 


CHAPTER     IV. 

BASALT. 

The  basalt  that  has  been  erupted  in  the  vicinity  of  Richmond  Mountain  and  to 
the  east,  forming  Basalt  Peak,  Strahlenberg,  and  Grater  Cone,  and  also  that  found  in 
the  neighborhood  of  Pinto,  though  varying  much  in  macroscopical  habit,  that  is,  in 
color,  density,  and  compactness,  and  in  its  occurrence  in  large  masses  or  thinly  fissile 
plates,  exhibits  in  thin  sections  under  the  microscope  the  greatest  uniformity  in  struc- 
ture and  in  the  microscopical  character  of  its  component  minerals.  It  has  many 
points  of  similarity  to  pyroxene-audesite  and  the  grounds  for  its  determination  as 
basalt  will  be  considered  when  the  nature  of  its  elements  has  been  described.  In 
general  it  consists  of  a  very  homogeneous  mixture  of  lath-shaped  feldspar  microlites 
and  angite  crystals  and  grains,  the  latter  being  in  excess,  with  a  smaller  amount  of 
hypersthene,  besides  which  are  minute  crystals  of  magnetite  in  a  more  or  less  abundant 
glass  base.  The  whole  is  of  very  even  grain  and  macroscopic  phenocrysts  are  almost 
never  met  with.  Olivine,  which  is  considered  an  essential  constituent  in  most  basalts, 
plays  the  part  of  a  very  inconstant  accessory  mineral  in  this  variety.  Since  the  micro- 
scopical habit  of  the  different  minerals  in  the  basalts  from  the  above  mentioned 
localities  is  constant  throughout  the  series  of  thin  sections,  a  single  detailed  descrip- 
tion of  them  will  be  sufficient. 

The  feldspar  is  triclinic,  in  lath-shaped  crystals  for  the  most  part  well  developed ; 
their  size  varies  considerably  within  certain  limits,  the  average  length  being  between 
0-1  and  0-0.">""",  a  few  reaching  0'25mm,  and  a  much  greater  number  being  microscopic- 
ally minute.  They  have  a  sharp  outline  along  the  base  and  brachypinacoid,  but  are 
less  regularly  terminated,  partly  squared  off  as  if  by  a  pinacoidal  face;  they  are  gener- 
ally notched  or  pronged,  appearing  as  if  made  up  of  several  prisms  of  unequal  length; 
frequently  the  halves  of  a  twin  are  separated  for  a  short  distance  at  either  end  of  the 
crystal  by  a  film  of  globulitic  glass.  The  smaller  individuals  show  but  two  twinning 
stripes,  but  in  the  stouter  crystals  more  are  present.  They  are  of  different  lengths, 
sometimes  wedging  out  in  the  middle  of  a  crystal.  A  second  twinning  at  nearly  90° 
to  the  first  is  seldom  seen,  except  in  some  of  the  stouter  individuals.  In  all  the  thin 
sections  where  the  feldspar  microlites  are  of  sufficient  size,  the  angles  of  extinction 
reach  those  of  anorthite  for  many  individuals,  which  also  show  a  very  high  light  in 
extremely  thin  sections  between  crossed  nicols,  indicating  that  a  portion  of  the  feld- 
spar belongs  to  that  species.  There  are  besides  many  more  faintly  polarizing  crystals 
386 


BASALT.  387 

with  low  extinction  angles,  which  may  most  likely  belong  to  a  less  basic  feldspar. 
The  long  narrow  crystals  are  without  zonal  structure  and  are  free  from  inclusions  of  any 
kind.  In  several  thin  sections  of  somewhat  more  coarsely  crystalline  structure  tin- 
shorter,  thicker  crystals  have  both  zonal  structure  and  numerous  globulitic  glass  inclu- 
sions. 

The  pyroxene  constituent  consists  of  both  augite  and  hypersthene.  The  angitc  in 
thin  section  is  almost  colorless  with  a  slight  tinge  of  yellowish  green.  The  hyper 
sthene  is  colored  light  green  and  light  reddish  brown  with  the  same  pleocbroism  as  that 
already  noticed  in  the  andesites,  a  phenomenon  more  common  in  the  larger  crystals, 
though  not  of  constant  occurrence  in  any  one  thin  section  and  frequently  confined  to 
the  inner  portion  of  a  crystal.  The  hypersthene  is  of  older  growth  than  the  augite, 
which  frequently  incloses  slender  prisms  of  the  former.  The  crystals  of  angitc  arc 
not  sharply  outlined,  except  in  a  few  of  the  larger  individuals,  but  have  an  uneven, 
jagged  outline  and  are  in  the  form  of  irregularly  terminated  prisms  and  grains,  with 
an  octagonal  cross-section,  which  is  well  defined  in  many  cases,  with  the  pinacoidal 
faces  more  highly  developed  than  the  prismatic.  It  has  a  good  cleavage  parallel  to 
the  latter,  with  an  occasional  less  perfect  jointing  parallel  to  the  former;  there  are  also 
irregular  transverse  fractures  across  the  long  slender  prisms.  The  larger  crystals  are 
sometimes  twinned  one  or  more  times  in  the  ordinary  manner  parallel  to  the  orthopin- 
acoid,  and  are  often  rich  in  glass  inclusions  with  a  g;is  bubble  and  sometimes  a  color- 
less microlite;  apatite  needles  are  not  met  with,  but  grains  of  magnetite  are  abundant. 
A  curiously  curved  crystal  of  augite  occurs  in  thin  section  260,  one  half  being  bent 
without  fracture  through  an  angle  of  40°.  Augite  is  the  most  abundant  mineral  com- 
posing these  basalts  and  is  considerably  in  excess  of  the  feldspar:  the  size  of  its  grains 
is  variable,  the  majority  ranging  from  0-01  to  0-05""";  many  are  smaller  and  a  large 
number  evenly  scattered  through  the  groundmass  average  0-1  """  in  diameter,  while  a 
small  number  of  porphyritically  developed  crystals  measure  0-75"""  in  length  and  are 
frequently  associated  in  groups  of  half  a  dozen  or  more.  Augite  is  also  found  in 
aggregates  of  radiating  prisms  encircling  macroscopic  grains  of  quart/.  1 1  is  in  nearly 
every  instance  perfectly  fresh,  but  in  thin  section  202  a  fibration  parallel  to  the  verti- 
cal axis  has  taken  place,  accompanied  by  a  red  coloration  around  the  margin  of  the 
crystal ;  the  fibers  polarize  brilliantly  between  crossed  nicols  and  extinguish  light 

parallel  to  their  length.  Bancroft  Libratf 

The  olivine,  which  appears  to  be  only  locally  developed  in  this  group  of  basalts 
and  is  found  in  only  a  few  thin  sections,  is  in  porphyritical  crystals  and  fragments, 
the  largest  not  more  than  0-7ranl  long  and  some  as  small  as  (M»5""n.  The  sections  are 
in  symmetrical  figures  of  four  and  six  sides,  and  also  in  irregular  shapes;  the  outline 
is  not  sharply  defined,  but  notched.  The  substance  of  the  olivine  is  eolorless  in  thin 
section  and  very  pure.  There  are  in  most  cases  t\vo  or  more  straight  cracks  parallel 
to  the  plane  of  the  optic  axes  and  the  usual  cleavage,  besides  numerous  fractures  in 


;>,SS  GEOLOGY  OF  THE  EUBEKA  DISTRICT. 

various  directions ;  frequently  curved,  presenting  the  appearance  common  to  pearlite 
structure,  a  beautiful  example  of  which  is  to  be  seen  in  Fig.  11,  PI.  in.  Inclu- 
.sions  are  very  rare,  the  only  kind  noticed  being  of  colorless  glass  with  a  fixed  gas 
bubble.  The  olivine  is  more  or  less  decomposed  in  every  instance,  the  alteration  pro- 
ceeding in  two  different  ways,  which  are  not  found  in  association  in  the  same  thin 
section.  One  is  the  characteristic  alteration  into  serpentine,  in  which  a  green  fibrous 
aggregate  is  formed,  the  fibers  projecting  normally  from  the  fractures,  in  which  is 
frequently  deposited  iron  oxide.  The  resulting  product  has  the  appearance  of  a 
network  with  meshes  of  variously  oriented  fibers,  which  are  at  times  so  intimately 
mixed  as  to  produce  aggregate-polarization.  In  thin  section  292  the  color  is  green, 
with  only  a  small  amount  of  reddish  yellow ;  but  in  thin  section  286  its  color  is 
brownish  green.  The  other  kind  of  alteration  may  be  seen  in  thin  sections  282, 
284,  269,  and  also  in  295,  296,  and  takes  place  in  a  different  manner.  There  com- 
mences from  the  surface  and  fractures  as  in  the  ordinary  process  a  fibration,  not 
in  directions  always  normal  to  the  surfaces  of  fracture,  but  in  lines  parallel  through- 
out the  entire  crystal,  and  parallel  also  to  some  direction  in  the  plane  of  the  more 
perfect  cleavage.  The  fibers  have  a  light  yellow  color  at  first,  which  deepens 
into  a  reddish  brown  or  blood  red  as  the  decomposition  proceeds;  they  polarize 
light  brilliantly  and  show  a  parallel  extinction  and  sometimes  a  faint  pleochroism. 
In  some  cases  there  appear  reddish  yellow  scales  and  thin  plates  and  a  general 
lamination,  and  less  frequently  the  lamination  or  fibration  is  altogether  wanting, 
when  the  section  yields  a  nearly  uniaxial,  negative  interference  figure,  the  plane  of 
the  optic  axes  in  the  other  cases  being  found  to  be  perpendicular  to  the  direction  of 
the  fibers.  The  alteration  in  some  individuals  has  started  from  the  center,  leaving 
the  outer  portion  still  fresh.  It  is  represented  by  Figs.  11,  12, 13,  PI.  in.  The  ordi- 
nary serpentine  alteration  product  is  sometimes  colored  the  same  orange  or  blood  red, 
but  is  easily  distinguished  by  its  internal  structure,  which  is  that  of  irregularly 
aggregated  fibers,  not  of  uniformly  parallel  fibers.  A  distinction  between  the  two 
has  not  been  made  by  Prof.  Zirkel,  for  in  his  Basaltgesteine  he  describes  a  reddish 
serpeutinization  of  olivine  from  the  basalt  of  Kotzhardt  in  the  Eifel,  and  afterwards 
a  form  of  decomposition  of  the  olivine  in  the  basalt  from  Steinheim  near  Hanau,  that 
corresponds  exactly  to  the  second  process,  just  described,  and  says  in  conclusion  that 
it  is  still  doubtful  whether  the  "  reddish  yellow  "  originates  immediately  from  the  fresh 
mineral  or  first  from  the  "green."1  And  again  in  his  report  on  the  microscopical 
petrography  for  the  Exploration  of  the  Fortieth  Parallel  he  remarks  that,  in  the  excel- 
lent basalt  from  east  of  Spanish  Spring  Station,  in  the  Virginia  Range,  "olivine 
occurs,  its  larger  crystals  altered  along  the  borders  and  cracks,  and  its  smaller  ones 
filled  with  a  brownish  red,  somewhat  fibrous  substance,  which  is,  without  doubt,  of  a 
serpentiuous  character."2  The  thin  section  of  this  rock  has  been  examined  and  the 

'!•.  Zirkel.    Basaltgeateine.    Bonn,  1870,  p.  65. 

'F.  Zirkel.    Microscopical  Petrography.     Washington,  1876,  p.  230. 


BASALT.  389 

red  alteration  found  to  belong  to  the  second  kind  of  decomposition,  one  section  of 
dark  red  altered  olivine  yielding  a  negative  interference  cross  with  OIK-  dark  ring. 

Prof.  Eosenbuscli  describes  a  similar  occurrence  in  the  melaphyre  from  Asweilen, 
among  the  crystalline  ingredients  of  which,  he  says,  "lie  large  grains  having  the 
appearance  of  specular  iron.  They  have  partly  the  form  of  oliviiie  and  show  by  well 
preserved  remnants  of  this  mineral  that  they  are  pseudomorphs  after  the  sa inc.  In 
other  cases,  however,  such  an  origin  is  not  demonstrable;  the  blood  red  substance  is 
then  either  very  compact  and  faintly  or  not  at  all  translucent  (basal  section)  or  else 
it  shows  a  perceptible,  monotonous  cleavage,  strong  pleochroism,  and  a  position  of 
the  axes  of  elasticity  parallel  and  at  right  angles  to  the  cleavage.  One  can  scarcely 
consider  this  body  as  anything  else  than  a  blood  red  mica,  for  1  know  of  no  such 
pleochroism  in  specular  iron." '  In  the  melaphyre  from  Reidelbacher  Hof  nea  r  Wadrill 
and  the  olivine-diabase  from  Eckelhausen  and  trotinesweilen  on  the  left  bank  of  the 
Rhine,  the  decomposition  of  the  olivine  has  resulted  in  the  same  red  micaceous  mineral. 
It  is  very  common  in  the  basalts  of  the  Fortieth  Parallel  collection,  as  noticed  by  Prof. 
Zirkel ;  but  after  a  careful  search  through  all  the  thin  sections  of  basalt  from  that 
region,  with  one  rather  doubtful  exception,  it  appears  that  the  two  different  proce-M- 
are  never  found  to  have  taken  place  together  in  the  same  thin  section.  The  resultant 
mineral  from  its  optical  properties  is  evidently  not  a  confused  aggregate,  but  a  crystal- 
lographic  individual,  with  parallel  orientation  of  all  its  parts,  for  the  extinction  of  light 
is  the  same  throughout  and  the  interference  figure  that  of  a  doubly  refracting  crystal. 

In  order  to  arrive  as  nearly  as  possible  at  its  actual  nature,  fragments  of  a  similarly 
altered,  porphyritic  olivine  in  the  basalt  from  Truckee  Valley,  Truckee  Range,2  were 
subjected  to  hot  concentrated  hydrochloric  acid,  and  afterwards  placed  under  the 
microscope,  when  they  were  seen  to  have  lost  their  intense  red  color,  which  was  due 
to  red  oxide  of  iron,  and  to  remain  light  yellow.  The  tabular  fragments  gave  for  interfer- 
ence figures  hyperbolas,  which  parted  only  a  short  distance,  indicating  a  small  angle 
between  the  optic  axes  and  showing  a  negative  bisectrix.  One  plate  was  marked 
by  lines  intersecting  at  00°,  leaving  no  reasonable  doubt  that  the  substance  in 
this  case  is  a  nearly  colorless,  mica-like  mineral,  colored  by  red  oxide  of  iron,  which 
latter  is  occasionally  seen  in  well  crystallized  hexagonal  films  in  the  cracks  of  less 
altered  olivine.  That  this  mineral  is  a  foliated,  crystallized  form  of  serpentine  seems 
probable  from  the  fact  that  most  of  these  basalts  are  so  fresh,  with  the  decomposi- 
tion of  the  olivine  frequently  confined  to  the  weathered  surface,  that  a  very  radical 
change  is  not  likely  to  have  taken  place,  and  that  a  simple  hydration  and  oxidation  of 
a  very  ferruginous  olivine  would  supply  all  the  chemical  elements  necessary  to  trans- 
form it  into  anhydrous  unisilicate  of  magnesia  and  ferric  oxide;  besides  which  is  flu- 
fact  that  the  optical  properties  of  the  mineral  in  question  correspond  to  those  given 

'H.  liosenlmsi'h.     Mikroskopi*. -In-  rii\»i<i|;ro]iliii<.     Stuttgart,  1877,  p.  400. 
'Forth-Ill  paniUcl  nilli-.-timi,  No.  2X129. 


390  GEOLOGY  OF  THE  EFKEKA  DISTRICT. 

by  Miller  for  thermophllite,  a  foliated  mineral  naviug  the  composition  of  serpentine, 
concerning  which  Prof.  Dana  remarks  that  it  seems  probable  both  that  this  is  "truly 
crystallized  serpentine"  and  that  "the  crystallization  of  this  species  is  actually  mica- 
ceous, like  that  of  chlorite  and  talc." '  The  red,  completely  altered,  macroscopic  olivine 
is  seen  in  the  hand  specimen  to  have  a  glistening,  mica-like  cleavage  surface.  There 
remained  in  the  portion  subjected  to  add  well  developed,  nearly  opaque  octahedrons, 
most  likely  picotite.  The  crystallization  of  the  olivine  appears  to  have  preceded  that 
of  the  other  minerals  by  only  a  short  period,  stopping  just  as  they  began,  for  no 
inclusions  of  them  are  found  in  the  olivine  except  around  its  border,  where  augite  and 
more  rarely  feldspar  are  seen  penetrating  its  surface,  causing  it  to  have  an  uneven  and 
broken  outline. 

A  less  important,  though  ever  present,  component  is  magnetite,  which  is  very 
abundant  in  well  crystallized  octahedrons  of  from  0-03  to  0-01 mm  and  less.  It  is  very 
uniformly  scattered  through  the  rock,  and  in  some  cases,  thin  sections  285,  282,  283, 
is  closely  associated  with  augite,  attaching  itself  to  the  surface  of  the  crystals  and 
being  included  in  them,  but  it  is  seldom  seen  penetrating  the  substance  of  the  feldspar 
crystals.  It  is  everywhere  fresh  and  there  is  no  evidence  under  the  microscope  of  the 
presence  of  titaniferous  iron.  As  accessory  minerals  the  only  one  to  be  mentioned  is 
quartz,  which  is  found  in  one  or  more  macroscopic  grains  in  nearly  every  hand 
specimen,  but  which  is  rarely  met  with  in  the  thin  sections.  There  is  a  fragment  in 
thin  section  261,  which  is  l-4mm  long,  with  an  angular  outline  and  conchoidal  fracture. 
It  is  cut  exactly  at  right  angles  to  the  optic  axis,  yielding  a  slightly  distorted  inter- 
ference cross  and  proving  to  be  a  single  individual  grain.  It  contains  a  crystal  of 
zircon,  several  hair-like  trichites,  and  a  few  irregularly  shaped  fluid  inclusions  with 
very  broad  dark  borders,  carrying  extremely  active  gas  bubbles,  which  disappear  at 
a  very  slight  elevation  of  temperature,  thus  indicating  the  fluid  to  be  liquid  carbon 
dioxide.  There  is  besides  these  inclusions  a  dihexahedral  cavity  filled  with  glass 
and  a  comparatively  large  crystalline  grain  which  crowds  the  bubble  of  gas  out  of  its 
usual  spherical  shape.  The  character  of  its  inclusions,  the  unity  of  the  whole  quartz 
as  a  single  individual,  together  with  the  surrounding  shell  of  augite  crystals,  leaves 
no  doubt  of  the  primary  nature  of  this  quartz,  which  corresponds  in  a  measure  to 
that  of  quartz-porphyry.  There  are  besides  in  some  thin  sections  amygdules  of  a 
green  or  red,  radially  fibrous,  delessite  like  mineral,  and  in  others  microscopic  aggre- 
gates of  one  of  the  zeolites,  whose  action  on  polarized  b'ght  and  apparent  polysyn- 
thethic  twinning  suggest  the  characteristics  of  chabazite  as  given  by  MM.  Fouque 
and  Michel-Levy. 

Glass  is  more  or  less  abundant  in  all  the  basalts  from  the  localities  mentioned.  It 
is  for  the  most  part  colorless  in  thin  section.  In  a  few  instances  it  is  brown,  and  in 
the  darker  varieties  it  swarms  with  seemingly  black  globulites,  which,  however,  are 

1 J.  D.  Dana.    A  System  of  Mineralogj,  5th  ed.,  p.  465. 


BASALT.  391 

found  to  be  transparent  with  very  dark  borders.  This  glass  base  and  the  micro- 
structure  of  the  rock  as  a  whole  are  the  only  variable  factors  in  this  otherwise  monot- 
onous group  of  rocks.  The  variations  in  these  features  will  be  separately  mentioned 
in  the  following  notes.  Thin  sections  253,  260,  261, 256, 257, 258,  and  262  are  from  the 
flow  of  basalt  cast  of  the  town  of  Eureka,  at  the  western  base  of  Eichmond  Mountain. 
No.  253  is  vesicular,  with  relatively  great  irregularity  in  the  size  of  the  crystals,  a  small 
amount  of  glass  base,  and,  what  is  generally  noticeable,  a  marked  flow  structure.  Nos. 
260  and  261,  which  are  closely  related  in  the  field,  have  a  more  even  grain  and  compar- 
atively little  glass,  with  few  globulites  and  acicular  microlites.  The  first  is  vesicular, 
with  some  cavities  filled  with  the  delessite-like  mineral  and  possibly  the  trace  of 
olivine.  The  second  is  compact;  the  mottled  appearance  of  its  groundmass  is  not 
traceable  to  any  modification  of  the  crystalline  structure  and  must  lie  in  the  isotropic 
base.  Nos.  256  and  258,  from  compact  and  vesicular  portions  of  the  same  flow  some 
distance  from  the  last  two,  are  alike  in  structure,  being  of  uneven  grain  with  numer- 
ous larger  augite  crystals  similar  to  No.  253.  The  glass  is  partly  brown,  which  is 
also  the  case  in  iMii  section  257,  which  is  exceptionally  beautiful,  having  a  coarser 
and  more  even  texture  with  a  larger  amount  of  glass  (Fig.  2,  PI.  vii).  Thin  section  262 
differs  from  all  the  rest  in  having  little  or  no  glass  and,  besides  the  lath-shaped  feld- 
spar microlites,  possessing  ill  defined  feldspar  grains  approaching  a  microcrystalline 
structure.  The  basalt  on  the  summit  of  Richmond  Mountain,  thin  sections  264,  265, 
267,  268,  has  the  same  modifications  of  its  structure.  Nos.  264  and  265  are  rich  in 
colorless  glass  filled  with  the  minutest  globulites,  which  give  it  a  brown  color.  The 
crystalline  constituents  are  very  small  and  almost  wholly  augite  and  magnetite.  The 
crystals  of  267  are  larger  and  include  more  feldspar,  which  is  also  true  of  268,  where 
the  mottled  appearance  of  the  rock  is  due  to  the  unequal  distribution  of  the  augite 
grains.  In  these  four  thin  sections  augite  is  greatly  in  excess  of  the  feldspar.  A 
fine  thin  section  with  rich  brown  globulitic  glass  and  minute  augite  crystals,  closely 
resembling  No.  264,  is  from  a  bowlder  on  the  foothills  to  the  north  of  the  mountain 
(270).  No.  269  is  a  red,  finely  porous  variety  from  near  the  summit  of  Richmond 
Mountain.  Its  red  color  is  the  result  of  the  partial  decomposition  of  abundant  olivine. 
The  rock  is  more  highly  crystalline  than  the  neighboring  basalt.  It  is  rich  in  feldspar 
and  poor  in  slightly  globulitic  glass.  Of  the  basalt  exposures  in  the  pumice  at  the 
south  base  of  Richmond  Mountain,  the  most  westerly  dike  exhibits  tine  columnar 
structure.  Its  thin  section,  271,  shows  it  to  be  quite  like  No.  260,  in  being  well 
crystallized,  with  little  glass  and  considerable  feldspar,  and  a  very  little  olivine. 
That  from  immediate  contact  with  pumice  on  the  spur  south  of  the  summit,  273,  is 
finely  vesicular,  poor  in  crystals,  and  full  of  ferrite,  resembling  those  portions  of 
basalt  found  included  in  several  altered  pumices.  Thin  section  274  is  from  one  of 
the  coarser  grained,  more  highly  crystallized  varieties,  which  is  poor  in  glass,  but 
very  rich  in  augite,  and  having  a  trace  of  olivine  in  the  form  of  a  small  number  of 
characteristically  decomposed  sections. 


392  GEOLOGY  OF  THE  EUREKA  DISTRICT. 

The  basalt  covering  the  large  area  east  of  Richmond  Mountain  is  represented  by 
three  thin  sections  from  Basalt  Peak,  283,  282,  and  284;  one  from  the  light  colored 
vesicular  variety  from  Strahlenberg,  285;  one  of  compact  rock  from  Basalt  Cone,  286, 
and  another,  288,  from  back  of  the  Toll  House  on  Newark  Valley  road.  The  first 
three  are  characterized  by  relatively  large  crystals,  and  a  coarsely  globulitic  glass, 
which  in  284  is  nearly  opaque,  being  of  a  dark,  rich  brown  in  the  thinnest  places. 
They  all  three  contain  olivine,  which  is  completely  decomposed  in  the  ordinary  manner 
in  283,  but  is  only  partially  altered  in  282  and  284  to  the  red,  laminated  sub- 
stance, already  described.  The  basalt  of  285  is  without  olivine  and  is  in  a  less  per- 
fectly crystallized  state.  The  colorless  glass  is  almost  free  from  globulites;  the  feld- 
spar inicrolites  are  small,  and  the  augite  is  in  much  larger  individuals  with  nearly  all 
the  magnetite  attached.  The  structure  of  286  is  identical  with  that  of  282,  the  glass 
is  coarsely  globulitic  and  there  is  a  small  amount  of  serpentinized  olivine  present. 
The  globulitic  base  of  288  is  crowded  with  feldspar  microlites,  with  many  micro- 
scopic porphyritical  augite  crystals,  and  numerous  grains  of  perfectly  fresh  olivine. 
Of  the  two  thin  sections  from  the  neighborhood  of  Pinto,  290  is  in  every  respect  like 
that  from  the  summit  of  Richmond  Mountain  (264),  and  contains  no  olivine,  while  292 
is  identical  with  the  basalt  of  Crater  Cone  (286),  and  abounds  in  altered  olivine. 

The  foregoing  detail  is  necessary  in  order  to  emphasize  the  fact  of  the  unity  of 
the  somewhat  scattered  outflows  of  this  rock,  as  it  shows  the  slight  and  nonessential 
character  of  the  variations,  the  fluctuating  percentage  of  the  olivine,  and  that  its 
presence  or  absence  is  without  influence  on  the  microstructure  of  the  rock.  The 
grounds,  then,  for  separating  this  rock  from  the  andesites  and  classifying  it  as  a  basalt 
may  be  summed  up  as  follows :  (a)  The  great  difference  in  microstructure  between  the 
andesite  of  the  district  and  this  rock,  which  is  not  porphyritically  developed  and 
which  is  very  glassy,  with  extremely  small  crystals  of  nearly  uniform  size,  having  no 
macroscopic  phenocrysts,  with  rare  exceptions;  (&)  that  while  the  feldspar  is  in  well 
developed  lath-shaped  crystals,  the  augite  is  mostly  in  less  regularly  outlined  crystals, 
and  in  much  greater  abundance,  occurring  frequently  in  larger  individuals,  in  addition 
to  which  there  is  a  small  percentage  of  hypersthene;  (c)  the  presence  of  oliviue,  though 
in  variable  quantities.  It  is,  however,  not  a  normal  basalt,  and  may  be  considered 
more  properly  an  intermediate  rock  between  basalt  and  pyroxene- audesite. 

Another  variety  of  basalt  is  found  in  the  southeastern  corner  of  the  district,  at 
Magpie  Hill  and  on  the  southern  slope  of  the  Alhambra  Ridge.  Thin  section  295  is 
from  the  former  and  shows  it  to  be  a  very  homogeneous  mixture  of  feldspar,  augite, 
olivine,  and  iron  oxide,  with  no  isotropic  glass ;  but  there  are  irregularly  scattered 
patches  of  a  light  purple,  cryptocrystalline  substance,  which  may  be  the  remains  of  a 
glassy  matrix.  The  rock  is  thoroughly  crystalline,  and  as  such  is  very  different  from 
the  first  described  variety.  The  body  of  the  rock  is  made  up  of  rather  broad,  inter- 
penetrating, lath-shaped  feldspar  crystals,  in  which  are  many  small  augite  prisms  and 


BASALT.  393 

grains  of  magnetite,  together  with  an  abundance  of  larger  and  better  developed 
olivine  crystals.  There  are  a  couple  of  grains  of  porphyril  ical  quartz,  and  calcite  is 
pretty  generally  disseminated  through  the  whole  mass.  The  feldspar  is  probably 
labradorite  with  some  little  anorthite.  It  is  not  well  outlined,  but  is  in  prisms,  elongated 
in  the  direction  of  the  brachydiagonal,  with  multiple  twinning  after  albite,  a  few  rare 
individuals  having  a  second  twinning  at  about  90°.  There  are  no  characteristic 
inclusions,  some  crystals  being  free  from  any,  others  containing  glass  and  small 
portions  of  the  associated  minerals,  besides  long,  slender  jointed  apatite  needles.  The 
augite  is  not  abundant  and  occurs  in  irregular  grains  and  in  short  prisms  with  pyra- 
midal termination.  It  is  of  a  light  green  color  without  pleochroism,  and  is  intimately 
associated  with  the  magnetite.  The  olivine  is  in  larger,  quite  characteristic  crystals, 
the  largest  lmm  long.  It  is  almost  completely  decomposed  to  the  blood-red  micaceous 
mineral  already  described,  and  is  in  every  respect  similar  to  that  found  in  the  basalt 
from  the  vicinity  of  Richmond  Mountain.  The  magnetite,  which  is  very  abundant,  is 
in  irregular  grains  and  crystals,  with  some  long,  narrow  forms,  that  suggest  titan- 
ifei'ous  iron. 

The  two  quartz  grains  in  295  are  very  interesting.  They  are  angular  fragments 
about  4mm  long,  with  distinctly  corroded  outlines  and  several  conchoidal  fractures; 
they  are  surrounded  by  shells  of  radiating  augite  prisms  and  calcite.  One  of  them 
is  represented  in  Fig.  4,  PI.  iv.  The  only  inclusions  are  a  few  dihexahedral  cavities 
containing  at  low  temperatures  liquid  carbon  dioxide.  The  bubbles,  which  have 
very  narrow  dark  borders,  are  motionless  at  a  moderately  low  temperature,  but 
become  greatly  agitated  at  about  60°  or  70°,  and  at  a  few  degrees  higher  tem- 
perature disappear,  reappearing  on  being  cooled  again.  The  quartz  is  evidently 
primary;  that  is,  antedates  the  final  consolidation  of  the  rock;  but  the  calcite, 
which  surrounds  it  and  penetrates  numerous  small  cavities,  and  also  occurs  in 
irregular  patches  through  the  groundrnass  of  the  rock,  is  secondary.  In  the  augite 
border  surrounding  the  quartz  grains  occurs  a  strongly  pleochroic  mineral  .in  short, 
stout,  well  developed  crystals.  It  is  biaxial  and  monoclinic,  and  two  cross-sections 
show  the  characteristic  form  and  cleavage  of  epidote,  with  the  strong  absorption 
parallel  to  the  clino-diagonal,  but  the  pleochroism  is  unusual  for  that  mineral,  being 
reddish  purple  parallel  to  the  axis  c,  light  yellow  to  colorless  parallel  to  a,  and  strong 
yellow  parallel  to  b.  This  is  the  pleochroism  of  pieduiontite  or  manganese  epidote, 
which  is  probably  the  mineral  present.  It  is  not  found  in  any  other  part  of  the  thin 
section. 

The  basalt  from  the  south  slope  of  the  Alhambra  Eidge,  thin  section  296,  is  simi- 
lar to  the  last  in  inicrostructure,  but  is  full  of  phenocrysts.  The  groundmass  is 
formed  of  the  same  interpenetrating  crystals  of  feldspar,  in  this  instance  labradorite, 
with  much  more  augite,  magnetite,  and  red  altered  olivine,  and  with  no  glass.  The 
larger  phenocrysts  are  not  sharply  outlined,  the  feldspars,  by  including  more  and 


394  GEOLOGY  OP  THE  EUEEKA  DISTRICT. 

more  of  the  other  minerals,  pass  gradually  into  the  surrounding  groundmass;  they 
have  numerous  gas  cavities,  some  containing  a  fluid,  besides  very  few  glass  inclu- 
sions. The  augite  is  in  part  so  clouded  with  dust-like  particles  as  to  be  almost 
opaque.  Hypersthene  occurs  among  the  larger  phenocrysts,  having  a  very  irregular 
form,  and  the  same  pleochroism  that  it  exhibits  in  the  andesite  of  this  region.  Asso- 
ciated with  the  phenocrysts  is  light  reddish  brown  mica,  which  has  a  small  angle 
between  the  optic  axes;  it  is  apparently  of  primary  origin.  There  are,  besides,  large 
grains  of  iron  oxide  and  considerable  calcite. 


DESCRIPTIONS  OF  PLATES.  395 

PLATE  in. 

1.   APATITE   FROM    HOKNBLKNDE-MICA-ANDE8ITB. 

Thin  section  37,  magnified  240  diameters.  Longitudinal  section  of  an  apatite  crystal,  showing 
prismatic,  pyramidal,  and  basal  faces,  and  bearing  glass  with  a  gas  bubble  in  a  negative  crystal 
cavity,  as  well  as  a  multitude  of  needle-like  inclusions  arranged  parallel  to  the  principal  axis  of 
the  crystal. 

2.    AUGITE   FROM   PYROXENE-ANDESITK   OF   CLIFF   HILLS. 

Thin  section  102  a,  magnified  55  diameters.  Section  showing  a  partial  black  border  of  magnitite, 
wholly  external  to  the  augite  crystal. 

3.   AUGITE  FHOM   PYROXENE-ANDESITE   OF  RICHMOND  MOUNTAIN. 

Thin  section  77,  magnified 55  diameters.     Section  showing  a  similar  black  border. 

4.   APATITE  FROM   HORNBLENDE-MICA-ANDESITB. 

Thin  section  37,  magnified  240  diameters.  Crystal  of  apatite  with  prismatic,  pyramidal,  and  basal 
faces,  showing  a  basal  cleavage  and  needle-like  inclusions  parallel  to  the  principal  axis  of  the 
crystal. 

5.    HORNBLENDE   AND  PARTLY   ALTERED  HYPERSTHENE    FROM  ANDES1T1C   PEARLITE. 

Thin  section  58,  magnified  75  diameters.  Section  of  a  fragment  of  hornblende  and  hyperstheue  in 
juxtaposition,  with  magnetite  and  glass  inclusions.  The  hornblende  has  remained  fresh  while  the 
hypersthene  has  been  partly  altered  into  fibrous  actinolite. 

6.   FELDSPAR  FROM  PYROXENE-ANDESITE. 

Thin  section  102,  magnified  25  diameters,  iu  polarized  light  with  crossed  nicols.  Section  of  a  triclinic 
feldspar  showing  a  marked  zonal  structure,  the  extinction  angle  at  the  center  being  25°  greater 
than  that  in  the  outer  zone.  There  are  very  narrow  strips  twinned  after  albite  that  do  not  show 
zonal  variation.  At  the  center  is  an  abundance  of  glassy  inclusions  that  appear  black  between 
crossed  uicols. 

7.   FELDSPAR  FROM  PINTO  RHYOUTE. 

Thin  section  122,  magnified  31  diameters,  with  crossed  nicols.  Section  of  a  triclinic  feldspar,  showing 
by  its  outline  that  it  is  a  Carlsbad  twin,  each  half  has  also  the  multiple  twinning  of  albite,  and 
each  a  different  set  of  extinction  angles.  It  is  cracked  transversely. 

8.    APATITE   FROM  ANDESITIC  PEARLITK. 

Thin  section,  75  a,  magnified  230  diameters.  Longitudinal  section  of  apatite,  showing  both  ends  ter- 
minated by  the  base  and  pyramid  and  having  well  defined  basal  cleavage,  with  a  small  amount  of 
needle-like  inclusions. 

9.    HYPERSTHENE  FROM  ANDESITIC  PEARIJTE. 

Thin  section  58,  magnified  40  diameters.  Partly  altered  hypersthene  with  inclusions  of  magnetite, 
glass,  and  small  apatite  crystals.  It  shows  the  fibrous  decomposition  taking  place  from  the 
extremities  and  fractures  of  the  crystal  and  proceeding  in  a  direction  parallel  to  the  principal 
axis  of  the  crystal. 


396  GEOLOGY  OF  THE  EUKEKA  DISTRICT. 

10.    ZIRCON    FROM    RIIYOL1TK. 

Thin  section  205,  magnified  100  diameters.  Cross  section  of  a  zircon  crystal,  showing  only  one  set 
of  prism  faces  and  a  cleavage  parallel  to  the  other. 

11.    OLIVINK   FKOM   BASALT   OK    BASALT   PEAK. 

Thin  section  282,  magnified  78  diameters.  Section  of  oil  vine,  showing  spheroidal  fractures  and  the 
irregular  outline  caused  hy  the  crystal  losing  itself  in  a  multitude  of  inclusions  of  augitc  and 
feldspar.  The  red  decomposition  is  seen  taking  placo  from  the  cracks,  giving  the  effect  of  a 
libration  that  is  parallel  throughout  the  whole  crystal. 

12,  13.    OLIVINK  FROM   ISASALT  OF   BASALT  PEAK. 

Thin  section  282,  magnified  75  diameters.  Sections  of  olivine,  showing  the  same  cieavage  and  decom- 
position as  Fig.  11,  in  one  the  alteration  setting  in  from  the  outside,  in  the  other  beginning  at 
the  center. 

14.    FELDSPAR   KKOM    KHYOLITE. 

Thin  section  145,  magnified  550  diameters.  Form  of  feldspar  microlitcs,  probably  oligochisc,  round 
crowded  together  in  a  glassy  rhyolite.  They  show  various  stages,  from  rectangular  compart 
crystals  with  sprouting  cornel's  to  the  finest  libers. 

15.   ZIKCON  FROM  ANDKSITIC   PKARLITK. 

Thin  section  54,  magnified  175  diameters.  Long,  slender  zircon  crystal,  O'S?1""1  long,  terminated  at 
both  ends,  lying  in  groundmoss ;  appears  to  have  the  form  ooP,  ooP»,  3P3,  with  P  and  Poo  . 

16.   ZIRCON  FROM  ANDKsVriC   PEARLITE. 

Thin  section  61,  magnified  455  diameters.  Long,  slender  crystal,  0-13  •'""  long,  lying  isolated  near  tin- 
edge  of  the  rock  section,  terminated  at  one  end  and  broken  at  the  other,  with  the  faces  ocp, 
ooPac,  and  3  P  3. 

17.   ZIRCON  IN   ANDESITIC  PKARLITK. 

Thin  section  75 b,  magnified  470  diameters.  Stont  crystal,  0-09 """  long,  lying  in  glassy  groundmass, 
with  apparently  ooP,  ooPoo ,  3  P3,  and  P  or  Pao  . 

18.    ZIRCON  FROM    RHYOLITE. 

Thin  section  168,  magnified  540  diameters.  Simple  crystal  of  zircon,  0'08nlm  long,  inclosed  in 
feldspar,  formed  of  a  single  prism  and  the  alternate  pyramid. 

19.  ZIRCON  FROM  ANDKSITIC   PKARLITE. 

Thin  section  58,  magnified  900  diameters.  Crystal  of  zircon,  0-044"""  long,  occurring  in  feldspar 
and  appearing  to  have  the  faces  ooP,  ooPoo,  3P3,  P,  and  Poo. 

20.  ZIRCON   FROM    AXDESITIC   PEARLITE. 

Thin  section  73,  magnified  570  diameters.  Crystal  of  zircon,  0-06mn>  long,  in  feldspar,  having  ooP, 
ooPoo,  3P3,  and  two  pyramids. 


U.S. GEOLOGICAL   SURVEY. 


GEOLOGY   OF   EUREKA    DISTRICT    PLATE  III 


" 

-*--.. 


14 


MICROSCOPIC     PETROGRAPHY. 


398  GEOLOGY  OF  THE  EUREKA  DISTRICT. 


PLATE  IV. 

1.   FRACTURES  ABOUT  GLASS   INCLUSIONS  IN   QUARTZ   OF  RHYOLITE  (CROSS  SECTION). 

Thin  section  111,  magnified  300  diameters.  A  cross  section  of  a  quartz  crystal  that  bears  glass  with  gas 
buhhles  in  dihexahedral  cavities.  About  each  inclusion  the  quartz  is  cracked  fora  short  distance 
in  three  planes,  corresponding  to  three  of  the  planes  of  symmetry  passing  through  its  vertical 
axis.  The  cracks  appear  as  six -rayed  stars  that  are  parallel  to  each  other  throughout  the  section. 
In  this  figure  a  number  have  been  brought  together  from  different  parts  of  the  same  quartz  sec- 
tion, in  order  to  show  the  different  appearances  when  the  inclusions  are  cut  through  the  middle 
or  near  one  end,  or  when  the  section  passes  just  above  or  below  them.  From  the  inclusion  nearest 
the  top  of  the  figure  it  will  be  seen  that  the  section  is  slightly  inclined  and  not  exactly  at  right 
angles  to  the  principal  axis  of  the  quartz  crystal.  The  illustration  shows  the  upper  end  of  this 
last-named  inclusion  and  the  lower  end  of  the  one  just  below  it  to  the  right. 

2.   FRACTURES  ABOUT  GLASS  INCLUSIONS  IN  QUARTZ   OF  KHYOLITE    (LONGITUDINAL  SECTION). 

Thin  section  111,  magnified  300  diameters.  A  longitudinal  section  of  a  quartz  crystal  only  1  millimeter 
from  that  in  Fig.  1,  showing  the  same  kind  of  glass  inclusions  with  vertical  fractures.  These 
are  drawn  as  they  occur,  without  any  change  of  position.  Those  lying  at  the  surfaces  of  the 
quartz  section  have  had  the  gas  bubbles  cut  in  grinding  and  filled  with  balsam. 

3.   QUARTZ-CONGLOMERATE. 

Thin  section  501,  magnified  33  diameters.  Section  of  fine  grained  conglomerate  with  siliceous  cement, 
showing  that  the  apparently  granitoid  quartz  grains  are  rounded,  water-worn  grains,  about 
which  the  silica  of  the  cement  has  crystallized  with  the  same  crystallographic  orientation  as  the 
nucleus,  thus  extending  the  individual  until  obstructed  by  the  surrounding  fragments. 

4.   QUARTZ   FRAGMENT  IN  BASALT  OP  MAGPIE   HILL. 

Thin  section  295,  magnified  28  diameters.  Section  of  crystalline  basalt,  showing  an  irregularly  shaped 
fragment  of  primary  quartz,  surrounded  by  a  shell  of  angite  crystals  and  patches  of  calcite. 
The  somewhat  darker,  broader  grains  in  the  augite  shell  are  piedmontite.  The  rock  is  composed 
of  feldspar,  minute  augite,  and  magnetite  crystals,  with  larger  crystals  of  red  altered  olivine. 


US  GEOLOGICAL   SURVEY 


GEOLOGY   Or   EUREKA    DISTRICT    PLA 


MICROSCOPIC     PETROGRAPHY. 


400  GEOLOGY  OF  THE  EUltEKA  DISTRICT. 


PLATE  V. 

1.   PLAGIOCLASE  FELDSPAR  IN   HORN'BLENDE-MICA-ANDESITE. 

Thin  section  42a,  magnified  45  diameters.  Between  crossed  nicols,  exhibiting  xona'  structure  and 
several  rounded  contours  due  to  partial  corrosion  at  different  stages  of  its  growth.  Also  a  net 
work  of  irregular  cracks. 

2.   A   MICROPEGMATITIC   PHEN'OCRYST  IN   RHYOLITE. 

Thin  section  141,  magnified  19  diameters.  Three  sanidine  crystals  surrounding  a  plagiochiso,  most  of 
which  has  fallen  out,  leaving  a  hole.  The  sanidine  is  filled  with  irregularly  shaped  shreds  of 
quartz  arranged  as  in  pegmatite.  Between  crossed  nicols.  The  quartz  is  dark,  the  feldspar 
light. 

3.    PLAGIOCLASE  FELDSPAR  FROM   HORNBLENDE-MICA-ANDESITK. 

Thin  section  35,  magnified  35  diameters.  Between  crossed  nicols,  exhibiting  polysyuthetic  twinning, 
zonal  structure,  and  microscopic  inclusions  of  glass. 

4.    PLAGIOCLASE   FELDSPAR   ADJACENT  TO   FIG.   8. 

• 

In  same  thu  section,  magnified  35  diameters.  Between  crossed  nicols,  exhibiting  polysyuthetic 
twinning  and  microscopic  inclusions  of  glass. 


u     5    GEOLOGICAL  Si  i 


LOGY  OF  EL 


MICROSCOPIC     PETROGRAPHY. 


402  GEOLOGY  OF  THE  EUKEKA  DiSTltiOT. 


PLATE  VI. 

1.   MICROPKGMATITIC   STIUTCTTKK    IX    GKANITK-POKI'HYRY. 

Thin  section  16,  magnified  100  diameters.     Intergrowth  of  quartz  and  feldspar.     The  quartz  is   the 
lighter  colored  portion  in  the  form  of  triangles  and  rhombs. 

2.   PLAGIOCLASK   FELDSPAR   IX   IIORXHLF.VDK-M1CA-ANDK8ITE. 

Thin  section  &5,  magnified  32  diameters.     Between  crossed  uicols,  exhibiting  the  parallel  growth  of 
twinned  individuals  and  zonal  structure. 


S    GEOLOGICAL  SURVEY 


MICROSCOPIC     PETROGRAPHY 


404  GEOLOGY  OF  THE  EUREKA  DISTRICT. 


PLATE  VII. 

1.    PHENOCRY8TS    OF   BLACK   BORDERED    HORNBLENDE    AND    PLAGIOCLASE   FELDSPAR   IN   HORNBLEXDK- 

BEADING   PYROXENE-ANDESITK. 

Thin  section  77,  magnified  50  diameters.     The  glass  inclusions  in  the  feldspars  and  tho  feldspar  micro- 
lites  in  the  groundmass  are  shown. 

2.  BASALT. 

Thin  section  257,  magnified  225  diameters.     The  lath-shaped  plagioclase  and  magnetite  grains  are  dis- 
tinctly shown,  but  the  augite  is  not  well  defined. 


QGY  OF  EUHEK  ,  PLATE 


MICROSCOPIC     RETROD 


406  GEOLOGY  OF  TUB  EUKEKA  DISTRICT. 


PLATE  VITI. 

1.   KIIYOI.ITK;  MicKospiiKiiri.rnc  WITH  A  MAKKKII  FLOW  sTurcTrRK. 

i 

Thin  section   130,  magnified  (ili  diameters.     'I'lic  angular  and  irregular  I'orin  of  the  phenocrysts  of 
c|iiart/  and  t'clilg|iar  sbo\v«  tin-  fractured  cliara(  tci. 

•1.  KIIYOI.ITK;  CILASSY  AND  IIANDKD. 
Thin  section  174,  magnified  14  diaiuetcr.s.     Small  phenocrysts  of  feldspar. 


U    S    GEOLOGICAL  SU 


OGY  OF  El 


V* 


MICROSCOPIC     P 


INDEX. 


A. 

Pago. 

Accessory  minerals  in  granite 338 

granite-porphyry 341, 345 

hornblcmle-nih-a  andesite 364 

lavas 262 

Acid  and  basic  magmas 254 

Adams  Hill,  description 117 

mines 290 

Aliatc  Pass 181 

Weber  conglomerate 92 

Alhambra  Hills 25 

basalt :!G4, 392 

description 153 

Allanite  in  granite 337,  338 

granite  porphyry 341 

lavas 262 

rhyolite 379 

rhyolitic  pumire 381 

Alpha  fault 158,160 

I Vak,  altitude 4 

region  of 159 

Ridge,  description 28 

Amphitheater  in  Water  Canyon 158 

Analyses  of  Washoe  rocks 282 

amlesitic  pcarlite 264 

basalt.  Basalt  Peak 264 

Richmond  Mountain 264 

Washoe 282 

<  'ambrian  iron  on' 37 

limestone,  Bell  shaft 37 

Pott's  Chamber 37 

Tip  Top  incline 37 

coal 98 

dacite ^  264 

Washoe 282 

granite-porphyry 228 

Hamburg  limestone 40 

hornblendf-amlfsitc,  Washoe 282 

liorublendr-niira-aiiili-sito 264 

Waslioc 282 

bypersthcne 356 

Kelly  ore 313 

Lone  Mountain  limestone 58 

Lord  Byron  ore 313 

I'oiionip  linifstoiir 49 

Prospect  -Mountain  limestone 37 

pyroxene-amh-.sitc 264.  266 

rhyolite 264 

\V  ashoe 282 

Ruby  Hill  ore all.' 


Ancient  coast  line 176 

Andesite,  alteration  products •     234 

Andesites  at  Washoe 2«2 

relative  age  of 278 

Andesitic  pearlite 234,368 

and  dacite,  modifications 871-373 

Anorthite  in  pyroxene-andesite 240 

volcanic  rocks 354 

Anticline  in  Fish  Creek  Mountains 210 

Newark  Mountain 156, 210 

Pinon  Range 201 

Prospect  Ridge 20 

Weber  conglomerate 162 

west  of  Wood  Cone 123 

Antimony  in  Kuby  Hill  ore 312 

Apatite  in  andesitic-pearlite  and  dacite 370 

granite 337, 338 

granite-porphyry 345 

hornblende-mica-andesite 367 

lavas 262 

pyroxene-andesite 359 

Argillaceous  beds  at  Eureka 178 

Arsenic  in  Kuby  Hill  ore 312 

Artemesia  tridentata 3 

A t rypa  Peak,  altitude 4 

section  across 65 

structure 22, 125 

Augite-andesite  (pyroxene-andesite) 233, 239 

Augite  in  audesitic  pearlite  and  dacite 370 

basalt 387, 394 

pyroxene-andesite 356 

B. 

Bald  Mountain  Coal  Company 97 

coal  seams 95, 97 

Banded  rhyolite 379 

Basalt  and  Rhyolite 285 

Basalt,  anjiitc  in 387, 394 

cutting  pyroxene-andesite 252 

distinguishing  characteristics 392 

feldspar  in 386, 393 

hypersthene  in 387, 394 

magnetite  in 390 

mica  in 394 

microscopical  characters 386 

mineral  composition 242 

olivine  in 387, 393 

penetrating  rhyolite 253 

piedmont ite  in 393 

pyroxene  in '   387 

407 


408 


INDEX. 


Page. 

Basalt,  quartz  in 390,393 

relative  age 276 

Basalt  Peak,  basalt 386 

Basaltic  glass 259 

Becker,  G.  F.,  on  olirino  in  basalt 257 

propylite 279 

p\  roxene-audesites  in  California 261 

Bennett  Spring,  section 187 

1  lit';. real  ion  of  Hoosae  fault - 16 

Biotite  in  andesitic  pearlito  and  dacite 370 

granite 337,338 

granite-porphyry 340 

hornblonde-mica-audesite 366 

pyroxenc-andrsite 359,  363 

rbyolite 376 

Bismuth  on  1'rospect  Kidge 313 

Blair.  Andrew  A.,  analysis  of  granite-porphyry 

Bold  Bluff,  description 160 

Bouncy.  T.O..  secondary  enlargement  ol'  quartz, 347 

British  Ainrricu.  Paleozoic  rocks  of 208 

Browns  Canyon 13" 

Bullion  product " 

Buusen.  Robert,  origin  of  lavas 273,  275 

C. 

Calcareous  beds  at  Eureka 178 

Hhalr.  base  of  Hamburg  limestone 44 

Cambrian  and  Silurian  Hocks !{4  -<i- 

species.  commingling  of 48,  5U 

fauna 41-47 

on  Koundtop  Mountain 112 

in  British  America 208 

rocks.  Prospect  Kidge : 34 

st-clioiis.  comparisons  of 117 

Carbon  Kidue 172 

audesitie  pearlitc  and  dacit'e  south  of  ...       368 

and  Spring  Hill  group 28, 165 

slructurc 30 

Carboniferous  at  l^uartz  Peak 197 

block,  depressed  body 28 

coal 95 

fauna  at  Quartz  Peak 199 

movement  iu 203 

Kocks 84-98 

species,  commingling  of 88 

thickness 13,  84 

Caribou  Hill 21 

Kureka  quart /He  54 

Pogonip  fauna 53 

Carlsbad  twins  of  plagioclase,  optical  properties  of.  ..360,  '•'•'>'.', 

Carson  Lake,  altitude 1 

Castle  Mountain 23 

Silurian  limestone 121 

Century  Peak  Kidge '•'>- 

I  Vrcoearpns  licdilblius 4,  24 

Chalcedony  in  andesite 234 

('barter  Tunnel 35, 100 

Chazy  fauna 50 

Chemical  composition  of  granite. porphyry 226,  228 

lavas 290 

<  'helming  fossils  at  Kurcka 71,  80,  89 

I  'l.issilicat  ion  of  lavas 233,  290 

Claudet,  Fred.,  analysis  of  ore 312 

Cliff  Hills 154 

and  Richmond  Mountain  compared 155 

plant  remains  in  "White  Pine  shale 155 

pyroxeue-audesite 23(1, 361 


Page. 
Coal  in  Carboniferous  .................................  95,  18! 

Coal-measure  fauna  overlying  coal  ....................        98 

Combs  Peak,  section  across  ........................  .'  .  .  .        65 

struct  ure  ................................       135 

thickness  of  Devonian  ...................        78 

Conical  Hill  ...........................................       168 

Copper  in  Ituby  Hill  ore  ..............................      312 

Corals  in  Century  1'eak  Kidge  .........................       152 

Cortex  Kangc  ..........................................       181 

County  Peak  region  ...................................      147 

section  ..................................        67 

across  ............................  68.78 

Crater  Cone  ...........................................      253 


basalt  of  ..................................  386 

Cross,  Whitman,  allauitean  accessory  mineral  .......  263 

on  hypersthenc-andesite  ............  241 

Cross-scction  in  Phcenix  Mine  ........................  306 

Curtis.  J.  S.,  course  of  Ruby  Hill  fault  ...............  303 

minerals  on  Ruby  Hill  ..................  311 

silver-lead  deposits  ....................  292.  301 

D. 
Dacite  ................................................  368,373 

-  a  t  Dry  Lak  e  ....................................      237 

South  Hill  ...................................      2.!7 

mineral  composition  ............................      236 

physical  features  ...............................      236 

relation  to  rhyolite  .............................      250 

IJana,  J.  D.,  cited  .....................................      390 

Uawsou,  Sir  J.  William,  on  Devonian  plants  ..........        69 

Descriptive  Geology  ..................................  99-1  74 

Devon  Peak,  structure  ................................      138 

Devonian  and  Carboniferous  rocks  ....................  63-98 

County  Peak  group  .....................        26 

at  Jones  Canyon  ............................      125 

Lone  Mountain  ..........................        74 

\VhitePine...  ...............       192 

Yahoo  Canyon  ...........................        83 

corals  in  Yahoo  Canyon  ....................      139 

distribution  of  .............................  203.  206 

fauna  .......................................        70 

Atrypa  Peak  ..........................  76,  125 

Brush  Peak  ...........................        76 

Combs  Peak  ..........................        76 

Mt.  Argyle  ...........................      193 

Newark  Mountain  ....................      156 

Quartz  Peak  ..........................      199 

Rescue  Hill  ...........................        80 

Woodpeckers  Peak  ...................        78 

in  British  America  ............  .  ............      209 

Devonian,  Lower  Carboniferous,  and  Coal-measure  spe- 

.      cies,  commingling  of  .....................        87 

plant  remains  ..............................        69 

rocks  ......................................  62-84 

thickness  ............................  13,  63 

Diamond  Mountains  ..................................  3,  26,  28 

Peak,  altitude  ...............................         4 

quartzite  at  Anchor  Peak  .............      146 

The  Gate  ................  142,145 

description  ..................        85 

evidence  of  age  .............       85 

thickness  ...................  13,85 

structure  .............................      157 

thickness  of  beds  .....................      158 

Range,  west  slope  ...........................      163 

Differentiation  of  lavas  ..................  267,  269,  287,  289,  290 

Dillur,  J.S.,  quartz  in  basalt  .........................      263 


INDEX. 


409 


Page. 

Dolomite  in  Hamburg  limestone 40 

Drown,  Thomas  M.,  analysis  of  pyroxene-andesite 2fl4 

Dry  Lake,  andcsitic  pcarlite  and  dacite  of 368 

Valley 141 

Durocher,  J.,  theory  of  lavas 273, 275 

E. 

East  Humboldt  Range 2,176 

granite  and  gneiss  in 219 

pre-Cambrian  barrier 176 

Emmons,  S.  F.,  Devonian  in  Tucubit  Mountains 202 

fault  in  Oquirrh  Kange 186 

fisli  remains  in  Tucubit  Mountains  ...  72 
pyroxene-andesite  cutting  hornblende- 

andesite 260 

Erian  plant  remains 70 

Erosion  of  Secret  Canyon  shale 39, 109 

White  Pine  shale,  Hayes  Canyon 157 

Eureka  a  volcanic  center 230 

and  Washoc  Districts  compared 230 

District,  bullion  product 6 

general  description 1-7 

history 6 

lead  product 7 

timber 4 

Mountains,  description 3 

quartzite,  base  of  Combs  Peak  24 

Caribou  Hill 212 

County  Peak  region 147 

description 54 

forfluxing  55 

Hoosac  Mountain 112 

Lookout  Mouiitaiu 130 

nature  of  material 180 

Pahrauagat  Range 196 

section  across 56 

section  at  Castle  Mountain 56 

Spanish  Mountain 24 

Surprise  Peak 108 

thickness 13,57 

WhitePine 191 

yielding  gold 55 

Tunnel 38,103 

F. 

Faulted  anticline.  Fish  Creek  Mountains 119 

Newark  Mountain 155 

anticlines 210 

block  in  Lamoureux  Canyon 134 

Faults- 
Alpha 159,160 

Hoosac 14, 15 

Jackson 51, 100, 300, 303 

Lamoureux 134 

Lookout 129 

M  in  I  MI- 141 

Newark 159 

Pinnacle  Peak 126,129 

Pinto 14,17,149 

ProspectPeak 15 

Rescue 14,18,151.154 

Ruby  Hill 17,101,302,307 

Spring  Valley  and  Sierra 14 

Fauna  of  the  Cambrian 41 

Devonian 70-84 

Lone  Mountain  limestone 59-61 

Lower  Coal-measures 86 


Page- 
Fauna  of  the  Pogonip 49-54 

Upper  Coal-measures 94 

Feldspar  in  andcsitic  pearlitc  and  dacitc 369 

basalt 386.393 

granite 337 

granite-porphyry 339 

hornblende-mica-andesite 364 

pyroxene-andesitc S49, 361 

rhyolite 375 

microlites  in  rhyolite 378 

Feldspathic  magma 254, 255 

Fish  Creek  Mountains 21-28 

granite-porphyry  iikes  in 122, 344 

Pogonip  fauna 53 

structure 118 

Fissure  eruptions 245, 270 

Fissures,  filling  of 308 

in  Prospect  Mountain  Tunnel 106 

Fossil  Butte 195 

Fossils— 

Acervularia  pentagona 138, 199 

Acrotreta  (like  A.  subconica) 60 

gemma 44,45,50,51,191 

Agnostus  bidens 43, 44, 45, 50, 108, 118. 192 

communis 43,44,45,50. 118,192 

interstrictus 42 

neon 43,44,45,50,118 

prolongus 45 

richmondensis 43, 118 

seclnsns 44 

tumidosns 45 

tumifrons 45 

Alveolites  rockfordensis 83 

Amboco3lia  umbonata 193, 194, 199 

A 1 1 1 1 1  h  i  M  1 1 51 

nevadensis 53 

Amplexus 199 

A  in  pi  il  l.u  i;i 167 

powelli 87 

Anadontopsis  amygdaUeformis 75 

Anarthrocanna 69 

Aneimites 70 

A  uomocare  parvum 42 

Archreocidaris 89 

Archffiocyathus 189 

atlanticus 189 

Arethusina  americanai 45,50,51 

Asaphns  caribouensis 50, 51, 52, 53 

curiosus 52 

platycephalus 59 

Athyris 85,199 

plano-sulcata 194 

angelica 82,84 

hirsuta 91 

roissyi 91. 96 

subquadrata 199 

subtilita 89,  94,  95,  96, 97, 98, 174 

Atrypa  desquamata 75, 77 

reticularis 64, 74, 75, 77, 78, 79,  80, 81, 

82,  83,  84,  120,  125,  132, 134,  138,  152, 
153, 184, 185, 193, 196, 198,  200, 202, 205 

Aalopora  gerpens 132 

Auricula 87 

Aviculopecten 82, 89 

afflnis 89,91 

catactas 193.194 


410 


INDEX. 


Fossils — Continued.  Page. 

A  viculopetiten  eurekensis 89 

haguei 89 

peroccidens 89, 171 

pintoensis 171 

Kathyrns 51 

Bathyuriscus  howelli 188 

producta 186,188 

Bathynrus  congeneris 123 

pogonipensis 195 

slinilis 52 

tuberculatus ' 123 

Bellerophon 52,91,197,199 

antiquatus 188 

Hurra 144 

majusculus 171 

neleos 77,193 

pelops 75,80 

perplexa 77, 132 

textilis 90 

Beyriehia ; . . .       52 

occidentalis 75,81.82,156 

Bryozoa 191 

Calonema  occidentalis 77 

Caniarophoria  eooperensis 91, 159 

Cardiola  fllicostata 90 

Cardiomorpha 82 

missouricnsis 193, 194 

Ceraurus 51,53,59,195 

Chastetes 51. 66, 94. 95, 9ti,  120, 133. 138 

Chariocephalus  tnmifrous 44. 1(12 

Chemung 80 

Ohonetes 81,82.193,199  \ 

deflecta 74.76,79.132 

fllistriata 74,  76 

granulifera 76, 89, 90, 96. 171, 199 

hemispherica 74, 76 

illinoiscn.-iis 194 

macrostriata 74.  76 

mucronata 81 

verneuiliana 89, 170 

Cladodna 72 

Cla<lopora 78 

pnlchra 83 

Coleolus  Icevis 83 

Coleoprion  minuta 61 

Conocardium 82, 193 

nevadensis • 77, 132 

Conocephalites 192 

Conocoryphe 189 

Crenipecten  hallanus  89, 171 

Crepicephalus  nitidus 192 

unisulcatus 192 

Cruziana 187 

Cryptonella  circula 75, 193 

piuonensis 80 

Ctenacantbns 72 

( '  Yuthophylluin 193, 194 

corniculum 83 

davidsoni 75 

rugosum 75 

Cyclonema  (like  C.  multilera) 77 

Cypbaspis 51 

brevimarginatus 52 

Cypricardinia 82 

indenta 77 

Cyrtina 198,199 

davidsoui 193 


Fossils— Continued.  Page. 

Cyrtina  hamiltonensis 199 

Cyrtoceras r>9 

<-*-ss;itor 194 

nevadeusis 84 

Cyrtolites  sinuatu« 61 

Cystidian  plates 52, 53, 60, 131, 191 

Cystipbyllum  americaimm 75 

Dalmauitcs 59 

tueeki 75, 77 

Dentalium 90 

Dicellocephalus  angustifrons 45 

bilobatus 45 

flnalis 50, 51 

inexpectaus 50, 51 

marica 45, 11 8 

nasutng 43, 44, 45 

osceola 44, 45, 46 

pepinensis 188 

ricbmondeiisis 44 

Diphypbyllum 96 

sinicoense 75 

Discina 51,74,96,169 

lodensis 193. 194 

minuta 70,  83, 84, 146.  169 

newberryi 89, 90 

Dyatactella  iusularis 132 

Ei -•culioinpbalus  (like  E.  ilistaus) 195 

devonicus 77 

Edmondia 74, 199 

medon 89 

[lifioneusis 75,  77,  78,  79 

Endoeeras  multitiibulatuni 52, 195. 197 

proteiforme 52,  53 

Eocystit«8  longidat'tyliis 188 

Etbinoiihyllnm  whitneyi ]8il 

Euoniphalus 84, 193, 199 

eurekensis : 77 

laxus 84, 193, 1%,  19!) 

(Straparollns)  opbinea 199 

subrugosus 89, 90 

Favosites 75,76 

basaltica 75,  76 

hemispherica 75 

Fenestella 82, 89, 90, 19:1, 194, 199 

Fusiliua  cyliudrica 93,  94,  95, 161, 164, 166, 168, 170 

robusta 94, 166 

Gomphoceras 90 

troniatites S2. 194 

desideratus 77 

kingii 194 

Goniophora  perangulata 75, 77 

Grammysia  arcuat:i 89 

hannibalcnsig 89 

niinur 80,84 

Graptolites 54 

Grapt<»lithus  bilidus 54 

Griffithides  portlovki 90, 91, 168, 171 

Halysites 1:1.  74. 136. 201, 205, 208 

catenulatus 59, 61,  64, 183.  l!ll 

Heliootoma 52,  60 

Holopea 196, 199 

Hyolithex 77, 194, 197 

blllingsi 187, 188 

primordialis 44, 46 

princeps 189 

vanuxemi 50 

lllccuus 59,61 


INDEX. 


411 


Fossils— Continued.  Page. 

I !  hi  •  1 1  us  r  ra  s- i  ra i n I  a 195,  197 

Illrenurus 192 

enrekensis 50,  52, 123, 191 

Iphidea  depressa 44 

Kiitor<.'ina  cingulata 189 

in  iniitis.siina 44,  45 

pan  n  u  la 188 

prospectensis 42 

wliitfleldi 43, 108 

Lamellibranchiates 72, 87. 88, 89 

Leipteria  rafinesqui 77 

T.eperditia 52,168,171,199 

bivia 52.  53,  128, 131, 195, 197 

rotnndatus 81 

Leptfflna  melita 50,51,123 

sericea 59 

Leptodesma  transversa 80 

Limoptera  sarmentica 77 

Liiigula  alba-pinensis 193, 194 

lama 74 

ligea 79,  80, 199 

]  ( mm  sis 74 

manticula 43, 44, 45, 46, 50, 51, 112. 118, 123 

mytaloides 90, 171 

white! 76 

Lingulella  ella ...186,188 

Lingulepis  inrera 44,  45.  50,  51, 123 

minuta 44, 45, 50, 192 

Loxonema 193,199 

approximatum 77 

nobile 75,77 

suhattenuata 77, 132 

Lunulicardiiim  fragoauni 193, 194 

Marlurea 51.53,61,134,195 

annulata 51,  52,  53,  61, 128 

rarinata 61 

suhanmilata 52, 195 

Macrocbeilns 168 

Macrodou  hamiltonae 89 

temiistriata 94, 164 

t  riiiii'atus 89 

Megambonia  oeeidualis 75 

Meristella  nasuta 75 

Metoptoma  devonica 79 

peroceidens 91, 168 

phillipsi 195 

Microdon  connatus 90, 171 

Tiiacrostriata , . .  75,  77 

Modiolnpsis  ueeideus 53,  60, 195 

pogonipcusis 53, 60, 195 

Modiomorpha K2, 196. 199 

altiforme 75 

ambigiia 89 

desiderata 89 

oblonga 77 

obtusa 75, 77 

pintoensig 171 

Monticulopora 52.  53, 60 

Murchisonia 60, 195, 197 

Myaliua  congeneria 91, 171 

uossus 89 

subovata 171 

si  i  In  i  nad  rat  a 94 

Mytilarea 77 

chemungensis 80 

dubia 75 

Naticopsis 84,90 


Fossils— Continued. 

Naticopsis  scquistriata 80 

Nucleospira  concinna 74, 199 

Nucula 89,168 

inRularis 89 

niotica 80 

rcscuensis 80 

Nuculites  triaugulus 193, 194 

Nyassa  parra 74. 79, 82 

Obolella 43 

ambigua 51 

discoidea 45,50,118 

pretiosa 43 

Ogygia  problematica 44 

Olenellus 183,186,187,208,209 

gUberti 42,46,186,187,189 

howelli 46,186 

iddingsi 42,187,189 

thompsoni 46, 189 

Vermont  ana 46 

Olenoides  qnadriceps 42 

spinosa 43 

typical!* 188 

Orthis 134,191,197 

eurekensis 43 

hamburgensis 51, 112, 123 

impressa 74,76,83,193,199 

lonensis 61 

macfarleni 73,79,193 

multistriata 202 

pecosi 94,96,98 

perveta 51,52, 53, 54, 61, 109, 131, 195 

plicatella 59 

pogonipensis 195, 197 

resupinata 89,91,96,171,199 

subquadrata 59,191 

testudinaria 51, 52, 54, 61, 112, 196 

triccnaria 53, 54, 109, 131, 195, 197 

tulliensis 74,82.83 

Orthoceras 52,  59,  75,  77,  84, 90, 168, 171, 195, 197, 199 

multicameratum 52, 53, 195 

randolpheusis 90, 171 

Pachypbyllum  woodmani 76,  83 

Paleomanon  roemeri 75 

Palseonello 82 

Paracyclas  occidentalis 74, 75, 80, 84 

peroccidens 193 

Pentamerus  comis 75, 77 

galeatus 1% 

lotis 193,196,199 

Phacops  rana 75, 77 

Pholidops  bellula 74 

quadrangular!* 74 

Physa  prisca 87, 167 

Pinna  consimilis 89, 171 

inezpectans 89 

Platyceras 199 

carinatiim 77 

carinatus 80 

conradi 77 

ili  •  n  t  :il  i  n  1 1 1  77 

in  >.  li  isii  1 1 1 75 

thetiforme 77 

thetis 77 

nndulatnm  77 

rial  yeti  isnia 193 

ambigna 84 

maccoyi K 


412 


INDEX. 


Fossils — Continued.  Page. 

Platyostoma 193 

lineata 77, 199 

Plethomytillis  oviforme 75 

Pleurotomaria 51, 53, 94, 131, 194, 198, 199 

conoideu 168 

lonensis 52,60,195 

nodoinarginata 90 

*  urbinifc  >nnis 94 

Plnmulites 51 

Polypora »4 

stragula 91 

Porambonites  obscurus 197 

Poaidomya  devomca 77 

Iffivis 77 

Productus 193,199 

cora 96,98 

costatus 96,170 

hallanus 74.79,80 

hirsutiforme 83,193,194 

lacbrymosa,  var.  lima 83 

longispinus 90,  .94, 161, 168, 170, 174 

navicella 74, 76 

nebrascensi 90,91,94,166,199 

prattenianus 89, 90, 91, 94, 95, 166, 168, 171 

punctatus 94, 170, 199 

semireticnlatus. . .  85,  89, 90,  91,  93,  94,  95,  96,  98, 
158, 161, 166, 168, 170, 171, 174, 180,  Ifl4, 199,  203 

Bhnmardianus 74,80,81,82,83,199 

shumardianus,  var.  py xidatns 74 

speciosus 83 

stigmatns 80,83 

subaculeatus 74, 76, 79, 80, 83, 193, 194 

tenuicostatas 199 

tmncata 76,79 

Prcetna .-.      194 

haldennani 80 

marginalia 75,77 

peroccidens 199 

Protospongia 192 

fenestrata 44 

Protypus  expanses 43 

senectns 43 

Pterinea 191 

flabella 75 

newarkensis 82 

pintoensis 171 

Pterinopecten 193 

hoosacensis 89 

spio 89 

Ptilodictya 53,171 

carbonaria M 

serrata 94, 168 

(Stenopera)  carbonaria 95, 164 

(Stenopera)  serrata 95 

Ptychaspis  miunta 45, 46 

Ptychoparia 42 

affinis 45, 50, 51 

annectang 51 

bella 44 

breviceps 45 

disBimffis 43 

granalosa 45, 50 

haguei 43,44,45,50,112 

Iffiviceps 44 

laticeps 44 

linnarssoni 44 


Fossils — Continued.  Page. 

Ptychoparia  minor 188 

occidentals 43 

oweni 43, 44,  50, 51, 108, 118 

pernasnta 44 

piochensia 187, 188 

prospectensis 42 

similis 44 

simulata 45 

unisulcata 44,45,50 

protoformis 89 

Raphistoma 52 

acnta 195 

nasoni 52, 53, 109. 128, 131 

Eeceptaculites 51 ,115, 120, 123, 124, 127, 134, 191 

ellipticus 52, 53 

elongatus 52,  53, 197 

mammillaris  .  .52, 53, 54, 60, 109, 131, 195, 197 

Retzia  mormoni 94,95,76,98 

radialis 193, 194 

Rhynchonella 59, 199 

(Leiorhyucbns  type) 89 

capax 191 

castanea 73, 74, 79, 80, 83 

dupiicata 80, 81, 193, 199 

emmonsi 193 

eurekcnsis 89, 90, 91, 171 

horsfordi 77 

laura 80,84 

nevadenais 80, 84 

occidcns 77, 132, 193 

pugnus 84 

qnadricostata 193, 194 

sinuata 80, 84, 199 

tetbys 75,77 

Sanguinolites  scolns 89 

combensis 77 

gracilis 77 

nsenia 90 

retnsuB 90, 171 

rigidus 84 

salteri 90 

sanduskyensis 77 

simplex 90 

striata 90 

ventricosuB 80 

Scenella  conula 42 

Schizambon  typicalis 51 

Schizodus  cuneatuB 90, 171 

deparcus 90 

orbicularis 75, 77 

pintoensis 171 

Scoliostoma  americana 77 

Skenidium  devonicum 74, 75 

Solenomya  curta 89 

Spirifera 77,78,193,190,199 

alba-pinensis 193 

annectans 91 

camerata 89, 90, 91, 94, 96, 161, 171, 174 

cristate 198, 200 

disjuncta 82,83,84,139,193 

engelmanni 79,80,81,82,84,193 

glabra 83,139 

leidyi 91 

lineata 169. 198, 199, 200 

(M.)  maia 74, 79, 80 

neglecta 89,91 


INDEX. 


413 


Fossils— Continued.  Page. 

Spirifera  pinguis 199 

pifioncnsis 74, 76, 82, 132, 138, 193 

pulchra 199 

raricosta 74 

rockymoutaua 91, 94, 96, 98 

(M.)  setigera 91 

striata 91, 171, 199 

strigosus 193 

trigonalis 91, 159 

nndifera 76 

vanuxemi 202 

varieosa 74 

Spiriferfna  cristata 94, 95, 394 

kentuckiensis 89, 96 

Stenotheca  elongata 43 

Straparollus 199 

newarkensis 82 

Streblopteria  similis 91,171 

Strephocbetus 189 

Streptelasma 59 

Streptorhynchus  chemungensis 76 

chemnngensis,  var.  pandora. .  .74, 76, 79 
chemungensis,  var.  perveraa  .  .74, 76. 79 

crenistria 89, 90, 96, 199 

filitexta 191, 196 

minor 61 

Stromatopora.  .66,  72,  74,  75,  76.  82, 83, 120, 138, 145, 171, 191, 

196, 198,  200 

Strophodonta 193, 199 

arcnata 74 

calvini 74,76 

oanace 132,193 

demlssa 76 

inoquiradiata 76, 193 

pattersoni 74 

porplana  . . . 74, 76,  82 

punctulifera 74, 76 

Strophomena  fontinalis 195 

nemea 52, 60 

perplana 198 

rhomboidalis 74, 79, 138 

Styliola  flssurella 74, 75, 79, 80, 81, 83, 84 

Subulites 195 

Syringopora 72, 164, 171, 199 

hisingeri 83,144 

perelegans 74,75,83 

Syringothyris  cuspidatus 91, 199 

Tellinomya  contracta 52,  53 

hamburgensis 51 

Tentaculitea  attenuates 77 

gracilistriatus 75, 80 

scalariformis .77, 132 

Terebratula 19.!,  199 

bovidena 94, 96 

bustata 91 

Thamniscus 193)  194 

Thecia  ramoaa 132 

Trematospira  infrcquens 74 

Trinucleus  concentricus 59, 191 

Triples!* 195 

calcifera 60,51,52,112,123.191 

Zaphrentis 60, 75,  76, 90, 94,  C5, 171, 196 

centralig 96 

Zaptychius  carbonaria 87 

Fossils  in  Itichmond  Mine 43, 118 

Fossils,  systematic  list  of 817-831 


Page. 

Fouqufi  and  Levy,  cited 350,353 

Fresh  water  shells  in  Carboniferous 87,166,181 

Fusilina  Peak,  region  of 159 

.thickness  of  Lower  Coal-measures 161 

G. 

Galena 3jo 

Garnet  in  granite- porphyry 341 

lavas 262 

rhyolite 377,379 

Geddes  and  Bertrand  dike 111,247 

Geological  cross-sections 211-21 7 

A-B 212 

CD-EF 21,101,213 

E-F 148 

GH-IK 215 

position  of  Eureka  quartzite 54 

range  of  ore  deposits 299 

section,  White  Pine 190494 

sketch  of  the  Eureka  District 8-38 

Georgia  fanna 43, 46 

Gilbert,  G.  K-,  Quaternary  valleys 33 

section  across  Timpahnte  Range 188 

Glendale  Valley,  hornblende-mica-andesite  of 368 

Gold  in  Eureka  quartzite 55 

Ruby  Hill  ore 312 

Gooch,  F.  A.,  analyses  of  dacite  and  rhyolite 282 

Granite 218,337 

absence  along  Diamond  Valley 219 

age  of. 116,220 

allanitein 337,338 

apatite  in 337,338 

biotitein 337,338 

bottom  of  Richmond  abaft 116 

feldspar  in 537 

hornblende  in 337, 338 

in  Great  Basin  ranges 219 

magnetite  in 337, 338 

Mineral  Hill 116,219 

quartz  in 337 

titanite  in 337, 338 

zircon  in 337 

Granite-porphyry 221 

allanite  in 341 

apatite  in 345 

biotite  in 340 

chemical  composition 228 

description 121 

dike,  Castle  Mountain 122 

dikes,  Fish  Creek  Mountains 122 

feldspar  in 339 

Fish  Creek  Mountains 344 

garnetin 341 

hornblende  in 341, 345 

microscopical  characters 339 

mineral  composition 226 

quartz  in 339 

quartz-porphyry  along  contact 226 

Graptolites  in  Pogonip  fauna 54 

Gray's  Canyon 131,133 

Peak,  region  south  of 133 

structure 22 

Great  Basin  Ranges,  description 2, 10 

structure 10 

Gronndmass  structure  of  pyroiene-andesite 241 


414 


INDEX. 


H. 

Page, 

Hall,  James,  Pogonlp  fauna 190 

Hamburg  limestone,  description 39 

dolomite 40 

thickness 40 

WhitePine 192 

Ridge,  ageof Ill 

description 110 

shale,  description 41 

thickness 41 

Hamilton  fossils  at  Eureka 71,  89 

Hart,  Edward,  analysis  of  basalt 264 

rhyolite 264 

Havalah  Range 9 

Highland  Range 186,189,194 

Olenellus  shale 46 

Hillebrand,  W.  F.,  analysis  of  coal 98 

ores 313 

Hoosac  fault 1*,  15 

bifurcation  of 16 

displacement 17 

Mountain 112 

andesitic  pearlite  and  dacite  of 368 

hornblende-mica-andesite  of 365 

Hornblende-andesite  and  pyroxene-andesite,  relative 

ageof 251 

Hornblende  in  andesitic  pearlite  and  dacite 369 

granite 337, 338 

granite-porphyry 341, 345 

hornblende-mica-andesite 365 

pyroxene-andesite 358,  363 

Hornblende-andesite  along  Hoosac  fault 244 

cutting  Hoosac  Mountain 1 13 

description  of. 233 

Dry  Lake  Valley 141 

Hornblende-mica-andesite 364 

apatite  in 367 

biotite  in 366 

feldsparin 364 

hornblende  in: 365 

magnetite  in 367 

microstrueture 367 

quartz  in 366 

zirconin 367 

HornitosCone 253 

tufl's  and  pumices '. . .  147 

Howell,  E.  E.,  Olenellus  shale  at  Pioche 186 

Humboldt  Lake,  altitude 1 

Hypersthene  and  augite,  isolation  of 242 

in  andesitic-pearlite  and  dacite 370 

basalt 387, 394 

dacite 236 

pearlite...  235 

pyroxene-andesite 356, 362 

wanting  in  normal  basalts 257 

I. 

Iddings,  J.  P.,  on  granite-porphyry 225 

lime-soda  feldspars 241 

metamorphosed  sandstones 144 

Modoc  section 65 

petrographical  features  of  pearlite..  236 

quartz  in  basalt 263 

structural  peculiarities  in  ground- 
mass  256 

zircon  crystals 262 

report  on  petrography 233 


Page- 
Igneous  rocks.  Sierra  Valley 131 

Inclusions  in  feldspar 355, 362, 365, 375 

Interstratified  conglomerate  in  Upper  Coal-measures. .  164 

Intrusive  dikes 243,247 

Prospect  Ridge 103 

Irving,  R.D.,  secondary  enlargement  of  quartz 347 

Isbister,  A.  K.,  Devonian  in  Mackenzie  river  valley  . .  73 

J. 

Jackson  fault 51.100.300,303 

Jones  Canyon,  Devonian  limestone 126 

Judd,  J.  W.,  propylite  of  Scotland 279 

Juniperus  occidentals 4 

K. 

Kuu.ii.  section 207 

Kaolinization  along  Hamburg  Ridge 310 

of  rhyolite 310 

Kawsoh  Range 232 

age  of  lavas 232 

King,  Clarence,  geological  position  of  rhyolite 249 

origin  of  igneous  rocks 277 

post-Jurassic  upheaval 9 

Quaternary  history 33 

relation  of  basalt  to  rhyolitt- 285 

Kinuicut,  Robert,  Devonian  in  British  America 73 

L. 

Labrauorite  in  pyroxeiie-andesite 240 

Lacustrine  deposits 32 

Lamellibranchiatas  in  Carboniferous 87, 169 

Devonian 72 

Lamoureux  Canyon 134 

section  east  of 67 

fault 134 

Lateral  compression  at  Eureka 210 

Lavas,  age  of '. 231 

along  faults 29 

lines  of  displacement 243 

Pinto  fault 149 

CarbonRidge 172 

common  source  of 267 

decomposition  along  fissures 235 

differentiation  of 267, 289.  287. 289, 290 

distribution  of 231 

ciicircliiii;  County  Peak  and  Silverado  mountain 

uplifts 246 

following  lines  of  least  resistance 245 

in  Grays  Canyon 245 

Great  Basin,  succession  of 288 

Sierra  Valley 245 

magmas  of  eruption 253 

mode  of  occurrence 243,249.289 

of  intermediate  composition 283.  288 

sequence  of 25:1, 276, 283, 290 

transitions  in 255 

welling  out  along  fissures 244 

Lead  in  Ruby  Hill  ore 312 

product 7 

Limestone,  Richmond  Mountain 167 

underlying  Cliff  Hills 155 

Liquid  carbonic  acid  in  quartzite 55 

Lone  Mountain,  beds  at  base  of  Combs  Peak 24 

description 60 

Nevada  fauna 62 

limestone,  description 57 

fauna 59 


INDEX. 


415 


Page. 

Lone  Mountain  limestone,  Niagara 57 

rnlmmagat  Range 196 

thickness 58 

Trenton 57 

Longitudinal  faults 14, 209 

Lookout  Mountain 22,129 

Eureka  quartzite 130 

Lower  Coal-measure  fauna 86 

fresli  water  fossils 87 

limestone 85 

thickness 85,161,170 

Spring  Hill 170 

Lower  Paleozoic  in  adjoining  regions 185 

Quaternary,  lacustrine  deposits 32 

M. 

Magnetite  in  basalt 390 

Cambrian  quartzite 107 

granite 337, 338 

hornblende-mica-andesite 367 

pyroxene-andesite 359, 363 

Magpie  Hill,  basalt 364,392 

quartz  in  basalt 263 

Mahogany  Hills 23,134 

Mabon,  R.  W.,  analysis  of  lava 264 

Marble  in  Prospect  Mountain  tunnel 106 

McConne.ll.  II.  G.,  structure  of  Rocky  Mountains 208 

McCoy's  Ridge 21 

Kureka  quartzite 54 

Lone  Mountain  limestone 114 

Pogonip 114 

Meek,  F.  B.,  fauna  from  Mackenzie  river 73 

Olenellus  fauna 46, 188 

Melville,  W.  H.,  analysis  of  andesitic  pearlite 264 

Metamorphosed  sandstones  in  Devonian  limestone..  .143,346 

Meteorological  record 5 

Mica  in  basalt 394 

Micropegmatitic  structure  in  granite-porphyry.. 341, 342, 344 

rhyolite 377 

Mirropoikilitic  structure 360 

Microscopical  Petrography  ot  Eruptive  Rocks 335-394 

Microstructure  of  basalt 391, 393 

bornblende-mica-andesite 367 

pyroxene-andesite 360, 363 

rhyolito 377 

Mineral  constituents  of  rbyolitic  pumice 381 

Mineral  Hill,  granite  body 107 

Mines- 
Albion  7,  302, 304 

Banner 295 

Bull  whacker 7, 221, 296 

California 314 

Champion  itiul   Iluekcye 6 

Connoly :)14 

Dugout 37,300 

Duntlurbcrg 7,  296,  309 

Eureka  Consolidated 7,  302.  305 

Fairplay 298 

Fourth  of  July 42 

Geddes  and  Bertrand 36. 37. 43, 108, 295. 309 

Hamburg 7, 296 

Hodgson 37 

Hoosac 113 

Industry 38 

Irish  Ambassador 108 

Jackson 7,37,49,51,190,301,302,304,309 


P»ge. 

Mines — Continued. 

KK 304 

Kelly 313 

Kentuck 298 

Lord  Byron 313 

Maryland 298 

Mountain  Boy 298 

Page  and  Corwin 296 

Phoenix 7, 301, 303, 304, 306, 307 

Price  and  Davies 296 

Queen 298 

Reese  and  Berry 297 

Rescue 298 

Richmond 6,7,36,108,302,304,305 

Seventy-six 297 

Silver  Connor 295 

Tiptop 6 

White  Rose 298 

Wide  Wide  West 296 

Williamsburg 7,  295 

Mining  tunnels  on  Prospect  Ridge 103 

Mixter,  W.  G.,  analyses  of  lava 282 

Modoc  fault 65,  75, 141, 142 

Peak,  section  across 65, 66 

section 16,24.141,142 

Molecular  changes  in  molten  masses 289 

Molybdic  acid  in  Ruby  Hill  ore 312 

Montezuma  Range 232 

Moore,  Gideon  E.,  analyses  of  lava 282 

Mountain  Quaternary 32 

shale 38.104 

thickness 38 

Mountains.    (Set  Peaks.) 

N. 

Ncolite.  a  natural  group  of  lavas 277 

Nevada  limestone,  Atrypa  Peak 125 

Century  Peak  Ridge 152 

description 63 

Newark  Mountain 156 

The  Gate 145 

thickness 64 

White  Pine 138 

plateau,  climate 2 

description I 

Newark  Canyon 28 

fault,  description .' 159 

Mountain  and  Diamond  Peak,  section  across.  158 

section  across 65 

structure 26, 155 

Ncwberry,  J.  S.,  varying  water  levels  in  mines 315 

New  York  Hill 165 

Niagara  in  Lone  Mountain  limestone 57,  59, 183 

1'iibranagat  Range 196 

White  Pine 191 

Wood  Cone 136 

O. 

Obsidian,  composition 169 

Oil -shore  deposits 177 

Olenellus  fauna  in  British  America 208 

shale 42,45 

Highland  Range 46,186- 

Oquirrh  Range 46. 186 

t  hickncss 42, 46 

OHvine  in  basalt 242. 2",  258. 387, 393 

relation  to  silica 258 


416 


INDEX. 


Page. 

Opal  in  andesite 234 

Oquirrh  Range,  Olenellus  shale 46, 186 

Ore,  age  of 300,316 

bodies,  age  of. 293 

Deposits 292-316 

Enby  Hill 301, 306, 311 

in  Carboniferous,  absence  of. 298 

Eateka  Tunnel 105 

on  Hoosac  Mountain 113 

Ores  deposited  as  sulphides 310 

earlier  than  basalt 293 

later  than  rhyolite 293 

of  Prospect  Kidge 300 

theCambrian 294 

Devonian 297 

Silurian 296 

Origin  of  lavas 272 

Orographic  blocks 10, 19 

P. 

Packer  Basin 131 

Pahranapat  Range 196-200 

Paleontological  classification 182 

Paleozoic  ocean,  western  limit  of 175 

Rocks,  Discussion  of 175-217 

in  British  America,  thickness 208  i 

Great  Basin 208  ! 

section,  Eureka 11-18  j 

thickness,  Cambrian 13 

Carboniferous  ...        13 

Devonian 13 

Silurian 13 

shore-line 175, 177 

Pancake  Ridge,  coal  seams 95 

Partial  magmas 287 

Peaks  and  Mountains — 

Alpha  Peak 159 

Anchor  Peak 146 

Argyle  Mountain 193 

Atrypa  Peak 65,75,76,125 

Basalt  Peak 25  j 

Bellevne  Peak 23,119,120 

Combs  Peak 65.75,76,78,135 

Devon  Peak 138 

Diamond  Peak 155. 157 

Dome  Mountain 147. 174, 244 

English  Mountain 149 

Fusilina  Peak 159,161 

Gray  Fox  Peak 110.  2:« 

Grays  Peak 133 

Honsac  Mountain 112 

Island  Mountain 80, 151 

Leader  Mountain 151 

Lone  Mountain 60 

Lookout  Mountain 22. 129 

Modoc  Peak 65, 75, 141, 142 

Moleen  Peak 181 

Newark  Mountain .  .65, 1 55 

Pinnacle  Peak 126,129 

Pinto  Peak 238 

Purple  Mountain 238 

Quartz  Peak 199, 204 

Ravens  Nest 200.203 

Round  top  Mountain Ill 

Sentinel  Peak 69 


Peaks  and  Mountains— Continued.  Page. 

Sign  al  Peak 141, 144 

Silver  Pe.ik 189 

Spanish  Mountain 140 

Strablc.  il*  rg 25, 162, 386 

Sugar  Loaf 69,79,83,151 

Surprise  Peak 108, 131 

Table  Mountain 138 

Temple  Peak 137 

White  Cloud  Peak 119 

White  Mountain 126 

Woodpeckers  Peak 65, 73, 78 

Penfleld,  S.  L.,  analysis  of  basalt 282 

Phillips,  J.  A.,  secondary  enlargement  of  quartz 347 

Phosphoric  acid  in  Cambrian  limestone 38 

Physical  classification  of  formations 184 

Piedmont  ite  in  basalt 393 

Pinnacle  Peak  fault 126,129 

PinonKange 200,323 

section  across 201 

The  Gate : 83,144 

Pinto  Basin,  basalt 386 

fault  displacement 18 

Peak  rhyolite 238,374 

Piute  Range 9 

Plant  remains  in  Carboniferous 87,167 

Great  Basin 180 

Hayes  Canyon 82 

White  Pine  shale 69 

Pliocene  age  of  lavas 289 

ores 316 

Pogonip  fauna 49 

CaribouHill 53 

Fish  Creek  Mountains 53 

Graptolites  in 54 

Lone  Mountain 60 

Surprise  Peak 53 

White  Mountain 52,127 

limestone,  description 48 

Pahranagat  Range 197 

west  of  Wood  Cone,  thickness 123 

White  Pine 48, 191 

White  Pine,  thickness 49 

Wood  Cone,  thickness 49 

Potsdam  fauna 43,46,49,183 

Pre-Cambrian  barrier  in  East  Humboldt  Range 176 

Pre-Tertiary  erosion 232 

Igneous  Rocks 218-229 

Price,  Thomas,  analyses  of  limestones 37 

Propylite,  ageof 276 

in  Great  Basin,  absence  of. 279 

Prospect  Mountain  limestone,  breccias  in 36 

description 36 

dolomite  in 36 

horizons  of  fossils 41 

marble  in 36 

Olenellus  shale 42 

Sierra  Valley 129 

stratification  in 36 

thickness 38 

quartzite,  absence  of  fossils 35.  41 

anticline 21 

description 35 

thickness 35 

tunnel 105 

fissures  in 106 

Peak,  altitude 4 


INDEX. 


417 


Page. 

Prospect  Peak,  fault 15 

Ridge,  Cambrian  rocks 21 

description 19 

section 102 

structure 20, 99, 107 

Pumico  and  Tuff 238 

Pumices  at  Hornitos  Cone 239 

Purple  Hill,  rhyolite 380 

Pyrites 310 

Pyroxene  in  basalt 387 

pyroxene-andesite 355, 362 

Pyroxcno-andcsite  and  basalt,  relative  age  of 252 

apatite  in 359 

augite  in 356 

Augusta  Range 260 

blotite  in 359, 363 

Cliff  Hills 154, 239,  361 

Cortez  Range 260 

feldspar  in 349, 361 

hornblende  in 358, 363 

hypersthene  in 356, 362 

in  Great  Basin,  age  of 260 

magnetite  in 359, 363 

mineral  composition 239 

Mt.  Davidson 281 

oli  vine  in 364 

pyroxene  in 355, 362 

quartz  in 360, 363 

Richmond  Mountain 233, 239,  348 

Shoshone  Range 261 

tridymite  in 360 

Truckee  Canyon 260 

tuff 385 

Wahweah  Range 260 

Pyroxenic  magma 254, 255 

Q 

Quartz  and  sanidine  in  rhyolite,  intergrowth  of 375 

conglomerate,  with  secondary  enlargement  of 

quartz  grains 346 

grains,  recrystallization  of 55 

in  andesitic-pearlite  and  (Incite 370 

basalt 390,393 

basic  lavas 263 

granite 337 

granite-porphyry 339 

hornblende-mica-andesite 366 

pyroxene-andesite 360, 363 

rhy  iilite 376 

Peak,  Pahranagat  Range 197 

Quartz  porphyry 220,345 

Adams  Hill 221 

Bullwhacker  Mine 221 

Ruby  Hill 117 

Quaternary  valleys 31 

R. 

Ravens  Nest,  Pinon  Range 200 

Ravines  in  Nevada  limestone : 148 

Red  Ridge,  Silverado  Hills 150 

Rescue  Canyon  rhyolite 237, 379 

fault 14, 18, 151, 154 

displacement 19 

1 1  ill 73, 79,  80, 152 

Rhyolitr.  absence  of  ferro-magnosian  minerals 266 

age  of 276 

MON   XX 27 


Page. 

Rhyolite,  allanite  in 262,379 

along  Hoosac  fault 244 

Jackson  fault 300 

Ruby  Hill  fault 244,:(04 

and  basalt,  differentiation  of 286, 291 

ore,  relative  age  of 309 

hiotite  in 376 

Brown's  Canyon 137 

clays 304 

cutting  Hoosac  Mountain 113 

dike  in  Dunderberg  Mine 309 

dikes  on  Prospect  Ridge :M7 

encircling  Alhambra  Hills 153 

feldspar  in 375 

feldspar  microlites  in 378 

garnet  in 282, 377, 379 

Mahogany  Hills 137 

microscopical  character 374 

mineral  composition 237 

penetrating  homblende-andesite 250 

physical  features 237 

plagioclase  in 376. 37fl 

Purple  Mountain 238.300 

quartz  in -176 

sanidine  in 375,379 

zircon  in 377 

Rhyolitic  pearlite 378 

pumice 380 

allanite  in 381 

in  contact  with  basalt 381-385 

Richmond  Mine,  Cambrian  fossils 43.118 

Mountain,  altitude 4 

and  Cliff  Hills  compared 155 

andesite 240,348 

basalt 252,386 

description 25. 240 

fauna 90 

junction  of  fault  lines 271 

limestone 167 

pyroxene-andesite 212 

rhyolitic  pumice 381 

source  of  lava 271 

Richthofen.  F.  von,  classification  of  rhyolite« 374 

origin  of  igneous  rocks 275 

Roberts  Peak  Mountains 3.203 

Rosenbusch,  H.,  cited 342.389 

Roth,  Justus,  origin  of  igneons  rocks 274. 275 

Roundtop  Mountain Ill 

Ruby  Hill  fault 17,101,302,307 

ore  deposits 301 

quartz-porphyry 117 

region 115 

underground  drainage  channels 315 

S. 

Salt  Lake,  altitude 

Sanidine  in  andesitic  pearlite 234 

rhyolit* 375.  379 

Schell  Creek  Mountains 47 

Secondary  dikes,  granite-porphyry 222 

fissureon  Ruby  Hill 305 

Secret  Canyon,  description 109 

shale W 

description 39 

thickness 39 

Sentinel  Peak 68.79 


418 


INDEX. 


Page. 

Serpentinization  of  oliviue 388 

Shah1  beds  in  Prospect  Mountain  limestone 38 

Shallow  water  deposits,  White  Pine  shale 69 

Sierra  Canyon,  andesitie  pearlite  and  dacite  of 368 

Pogomp  fauna 130 

fault 20 

Valley,  a  center  of  eruption 131 

Prosjicct  Mountain  limestone 129 

Signal  Peak,  stmcture 144 

Silica  in  basalt i'.!l 

Siliceous  lii'da  at  Eureka 178 

Silurian  and  County  Peak  group 26 

at  Highland  Range 194 

Quartz  Peak 199 

in  Kritisb  America 208 

Pahranagat  Range 196 

Valley 195 

rocks   34.47 

Silverin  Ruby  Hill  on- 312 

Peak.  Olenelltts  .shale 46 

Silverado  and  County  Peak 25, 148 

Silverado  Hills 149 

Soil 5 

Sorby.  If.  ('.,  secondary  enlargement  of  quartz 347 

Spanish  Mountain 24 

structure 1 40 

Spherulites  in  andesitic  pearlite  and  dacite 371 

Spring  Hill,  grouping  of  fossils 88 

region  of 171 

structure 168 

synclinal  fold 29 

Valley 134 

and  Sierra  fault 14 

St.  John  fauna 46 

SI  ampede  G ap 187 

St  rahlenberg 25 

basalt 386 

Stratified  limestone,  Richmond  Mine 37 

Structural  modifications  of  granite-porphyry 342.  343,  344 

features 209-217 

variations  in  granite-porphyry 225 

Structure  of  Timpahute  Range 188 

Weber  conglomerate 162 

Sugar  Loaf 69.83.151 

Sulphides  filling  fissures 294 

on  Ruby  Hill 310 

Surprise-  Peak 108 

Pogonip  fauna 53 

Synclinal  fold,  Spring  Hill 29 

Syncline  in  Weber  conglomerate 162 

on  Combs  Peak 136 

west  of  Wood  Cone 122 

T. 

Table  Mountain,  structure 138 

Tellurium  on  Prospect  Ridge 313 

Temple  Peak,  structure 137 

Tertiary  age  of  eruptions 232 

and  post-Tertiary  Volcanic  Rocks 280-291 

lavas 30 

The  Gate,  Piflon  Range 83, 144 

Thickness  of  beds  at  Diamond  Peak 158 

Carboniferous 13, 84 

Devonian 13,  63 

Combs  Peak 78 

Diamond  Peak  quartzite 13,85 

Eureka  quartzite 13,57 


I'age. 

Thickness  of  Hamburg  limestone 13, 40 

shale 13,41 

Lone  Mountain  limestone 13, 58 

Lower  Coal- measure  limestone. . .  .13.85, 161, 170 

measures,  Carbon  Ridge 173 

Spring  Hill 170 

Mountain  shale 13, 38 

Nevada  limestone 13,64 

Olenellus  shale 42. 46 

Paleozoic  rocks  in  British  America 208 

Pogonip  limestone 13, 49 

west  of  Wood  Cone 123 

WbitePine 49 

Wood  Cone 49 

Prospect  Mountain  limestone 13, 38 

quartzite 13, 35 

Secret  Canyon  shale 13,39 

Upper  Coal-measures 13. 93. 164 

MoleenPeak 93 

Weber  conglomerate 13, 92 

Carbon  Ridge 173 

White  Pine  shale 13, 154 

Timber  growth ,  Eureka  Mountains 4 

Tinipahute  Range 188 

Olenellos  shale 46 

Tin  on  Prospect  Ridge 313 

Titanite  in  granite 337,338 

Tiirnebohm,  A.  E.,  secondary  enlargement  of  quartz..      347 

Trachyte,  age  of 276 

Trachyte  in  Great  liasin,  absence  of 280 

Trenton  at  White  Pine 191 

fauna 24,50,57,59,183 

horizon 136 

in  Lone  Mountain  limestone 57 

Pahranagat  Range 196 

Tridymite  in  pyroxene-andesite 263, 36U 

Tucubit  Mountains 202, 203 

Turner,  H.  W.,  andesite  in  California 261 


Unconformity  between  Carboniferous  and  Triassic. ...  10 

in  Silurian 202 

Upper  Coal-measures 164 

Upper  and  Lower  Devonian,  commingling  of 148 

comparisons  of 73 

Upper  Coal-measure  fauna 94 

limestone,  area  of 164 

Upper  Coal-measures,  Moleen  Peak,  thickness 93 

thickness 164 

Upper  Helderberg  fossils  at  Eureka 71, 132 

Upper  Quaternary 32 

sub-aerial  deposits 32 

Upper  Silurian,  distribution  of 203, 206 

Uranium,  Prospect  Ridge 313 

V. 

Van  Hise,  C.  R.,  secondary  enlargement  of  quartz 347 

Variations  from  normal  basalt 286 

rhyolite 2X6 

in  lavas 274 

Volcanic  action,  history  of 268-272 

activity,  duration  of 232 

rocks,  age  of  eruptions 231 

microscopical  characters 348 

natural  succession  of 278 

relative  age  of 249 


INDEX. 


419 


. 

Page. 

Walcott,  C.  D.,  Carboniferous  fauna  at  Diamond  Peak..  159 

fauna  of  Spring  Hill 

White  Pine  shale 70, 71, 82 

fresh-water  fauna  in  Carboniferous 87,167 

Graptolites  in  Pogonip 54 

Olencllus  fauna 189 

placoganoid  fishes,  Kanab  Canyon .  . 

section  across  Lone  Mountain 61 

In  Kanab  Valley 207 

Silurian  in  Highland  Range 195 

at  White  Pine 190.191 

structure  of  Highland  Range 187 

Systematic  list  of  Fossils 317 

Waltershausen,  Sartorius  von,  origin  of  igneous  rocks  .  275 

Wahweah  Range 

Wasatch  Range,  section  in 

sequence  of  strata 175 

section 206 

Washoe  and  Eureka  Districts  compared 

Waverly  fauna  at  Eureka 89 

Weber  conglomerate.  Agate  Pass 92 

Alpha  Ridge 

Carbon  Ridge 92,173 

deposits  in  shallow  water 181 

description 91 

structure 

thickness 92 

Peak  and  Pinto  Springs  region 161 

conglomerate 30 

West  Humboldt  Range 9 

White,  C.  A.,  Cambrian  fauna,  Schell  Creek  Mountains  47 

commingling  of  Carboniferous  species  .  -  88 

White  Cloud  Peak 119,120 

altitude 4 

White  Mountain,  altitude 126 

description 126 

Pogonip  fauna 52 

White  Pine,  geological  section 190 

Mountains 3 

shale 153 

CliffHUls 154 

description 68 

Hayes  Cany  on 157 


Page. 

White  Pine  shale,  Newark  Mountain,  section  across. . .  82 

section  across 81 

thickness 69. 154 

WhitePine 194 

thickness  of  Pogonip  limestone  at 49 

Whitneld,  J.  E.,  analysis  of  basalt 264 

iron  ore 107 

Whitneld,  R.  P.,  commingling  of  Carboniferous  species  88 

Pogonip  fauna 190 

Whitney,  J.  D.,  fossils  from  Silver  Peak 189 

Wide  West  ravine  eroded  in  Hamburg  shale 118 

Wild  Cat  Mountains 202 

Wood  Cone,  ridge  west  of 122 

thickness  of  Pogonip 49 

Woodpeckers  Peak 73 

altitude 4 

section  across 65 

Woodward,  R.  W.,  analyses  of  lava 282 

Wulfenite 311 

T. 

Taboo  Canyon 139 

Devonian 83 

rhyolite 378 

Yellowstone  National  Park,  hornblende-andesite  ear- 
liest eruptive  rock 285 

thermal  activity 309 

Young,  A.  A.,  secondary  enlargement  of  quartz 347 

Z. 
Zinc  In  Ruby  Hill  ore  ...  312 

Zircon  in  andesitic  pearlite  and  dacite 370 

granite 337 

hornbleiide-micu-audesite 262.  367 

lavas 262,284 

rhyolite 377 

Zirkel,  F.,  alteration  of  olivine 388, 389 

felt-like  structure  in  andesites 360 

igneous  rocks  of  Fortieth  Parallel  Explora- 
tion   354,357 

lavas  in  the  Wahweah  Range 260 

olivine  in  basalt 390 

structure  of  pyroxene-andesite 241 

Zonal  structure  of  feldspars 354, 362, 364 


